See also:

• SPECTROSCOPY (from Lat. spectrum, an appearance, and Gr. a-sore y, to see)

SPECTROSCOPY (from See also:

• LAT

Lat. spectrum, an See also:

• APPEARANCE (from Lat. apparere, to appear)

appearance, and Gr. a-sore y, to see)  , that See also:

• BRANCH (from the Fr. branche, late Lat. branca, an animal's paw)

branch of See also:

• PHYSICAL

physical See also:

• SCIENCE (Lat. scientia, from scire; to learn, know)

science which has for its See also:

• PROVINCE
• PROVINCE (Lat. provincia; perhaps a contraction of providentia)

province the investigation of spectra, which may, for our See also:

• PRESENT

present purpose, be regarded as the product of the See also:

• RESOLUTION

resolution of composite luminous radiations into more homogeneous components . The See also:

• INSTRUMENTS

instruments which effect such a resolution are called spectroscopes . r . See also:

• INTRODUCTORY

Introductory.—The announcement of the first discoveries made through the application of See also:

• SPECTROSCOPY (from Lat. spectrum, an appearance, and Gr. a-sore y, to see)

spectroscopy, then called spectrum See also:

• ANALYSIS (Gr. avci and Meta, to break up into parts)

analysis, appealed to the See also:

• IMAGINATION

imagination of the scientific See also:

• WORLD

world because it revealed a method of investigating the chemical nature of substances independently of their distances: a new science was thus created, inasmuch as chemical analysis could be applied to the See also:

• SUN
• SUN (0. Eng. sonne, Ger. sonne. Fr. soleil, Lat. sol, Gr. ijXior, from which comes helio- in various English compounds)

sun and other stellar bodies . But the beautiful simplicity of the first experiments, pointing apparently to the conclusion that each See also:

• ELEMENT (Lat. elementum)

element had its characteristic and invariable spectrum whether in the See also:

• FREE

free See also:

• STATE
• STATE, GREAT OFFICERS OF

state or when combined with other bodies, was soon found to be affected by complications which all the subsequent years of study have not completely resolved . See also:

• COMPOUND
• COMPOUND (from Lat. componere, to combine or put together)

Compound bodies, we now know, have their own spectra, and only when See also:

• DISSOCIATION

dissociation occurs can the compound show the rays characteristic of the element: this perhaps was to be expected, but it came as a surprise and was not readily believed, that elements, as a See also:

• RULE

rule, possess more than one spectrum according to the physical conditions under which they become luminous . Spectrum analysis thus passed quickly out of the See also:

• STAGE (Fr. 6/age; from Lat. stare, to stand)

stage in which its See also:

• MAIN (from the Aryan root which appears in " may " and " might," and Lat. magnus, great)
• MAIN (Lat. Moenus)

main purpose was " analysis " and became our most delicate and powerful method of investigating molecular properties; the old name being no longer appropriate, we now speak of the science of " Spectroscopy."' Within the limit of this See also:

• ARTICLE (from Lat. articulus, a joint)

article it is not possible to give a See also:

• COMPLETE

complete See also:

• ACCOUNT
• ACCOUNT (through O. Fr. acont, Late Lat. comptum, cornputare, to calculate)

account of this most intricate branch of physics; the writer therefore confines himself to a See also:

• SUMMARY

summary of the problems which now engage scientific See also:

• ATTENTION (from Lat. ad-tendo, await, expect; the condition of being " stretched " or " tense ")

attention, referring the reader for details to H . See also:

• KAYSER, FRIEDRICH HEINRICH EMANUEL (1845- )

Kayser's excellent and complete Handbuch der Spectroscopie . 2 . Instrumental.—The spectroscope is an See also:

• INSTRUMENT (Lat. instrumentum, from insincere, to build up, furnish, arrange, prepare)

instrument which allows us to examine the vibrations sent out by a radiating source: it separates the component parts if they are homogeneous, i.e. of definite periodicity, and then also gives us the See also:

• DISTRIBUTION (Lat. distribuere, to deal out)

distribution of intensity along the homogeneous constituents . This resolution into See also:

• SIMPLE

simple periodic waves is arbitrary in the same sense as is the decomposition of forces along assumed ' The present writer believes that he was the first to introduce the word " Spectroscopy " in a lecture delivered at the Royal Institution in 1882 (Proceedings, vol. ix.) . axes; but, in the same way also the results are correct if the resolution is treated as an See also:

• ANALYTICAL

analytical See also:

• DEVICE

device and in the final result account is taken of all the overlapping components .

Spectroscopes generally consist of three parts: (r) the collimator; (2) the analysing appliance, (3) the See also:

• TELESCOPE

telescope . The slit of the collimator confines the See also:

• LIGHT

light to a nearly linear source, the See also:

• BEAM
• BEAM (from the O. Eng. beam, cf. Ger. Baum, a tree, to which sense may be referred the use of " beam " as meaning the rood or crucifix, and the survival in certain names of trees, as horn-beam)

beam diverging from each point of the source being subsequently made parallel by means of a See also:

• LENS
• LENS (from Lat. lens, lentil, on account of the similarity of the form of a lens to that of a lentil seed)

lens . The See also:

• PARALLELISM, PSYCHOPHYSICAL

parallelism, which is required to avoid aberrations, otherwise introduced by the See also:

• PRISM (Gr. rrpivµa, properly a thing sawn, Irpii"ety, to saw)

prism or grating, may often be omitted in instruments of small See also:

• POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power . The lens may then be also dispensed with, and the whole collimator becomes unnecessary if the luminous source is narrow and at a See also:

• GREAT

great distance, as for instance in the See also:

• CASE
• CASE, JOHN (d. 1600)

case of the See also:

• CRESCENT (Lat. crescens, growing)

crescent of the sun near the second and third contact of a See also:

• TOTAL

total See also:

• SOLAR, SOLLER (Lat. solarium, Fr.• galetas, Ital. solaio)

solar See also:

• ECLIPSE (Gr. €uheo,tis, falling out of place, failing)

eclipse . The telescope serves to examine the See also:

• IMAGE (Lat. imago, perhaps from the same root as imitari, copy, imitate)

image of the slit and to measure the angular separation of the different slit images; when photographic methods are employed the telescope is replaced by a See also:

• CAMERA (a Latin adaptation of Gr. Kagapa, an arched chamber)

camera . The analysing appliance constitutes the main feature of a spectroscope . It may consist of one of the following: a . A prism or a See also:

• TRAIN (M. Eng. trayn or trayne, derived through Fr. from Late Lat. trahinare, to drag, draw, Lat. trahere, cf. trail, trace, ultimately from the same source)

train of prisms . These are employed in instruments of small power, especially when luminosity is a See also:

• CONSIDERATION (from Lat. considerare, to look at closely, examine, generally taken to be from con-, and the base seen in sidus, sideris, a star, the word being supposed to be originally an astrological or astronomical term)

consideration; but their See also:

• ADVANTAGE

advantage in this respect is to a great extent lost, when, in See also:

• ORDER
• ORDER (through Fr. ordre, for earlier ordene, from Lat. ordo, ordinis, rank, service, arrangement; the ultimate source is generally taken to be the root seen in Lat. oriri, rise, arise, begin; cf. " origin ")
• ORDER, HOLY

order to secure increased resolving power, the See also:

• SIZE

size of the prisms, or their number, is unduly increased . b . A grating . Through H .

A . See also:

• ROWLAND, HENRY AUGUSTUS (1848-1901)

Rowland's efforts the construction of gratings has been improved to such an extent that their use is becoming universal whenever great power or accuracy is required . By introducing the See also:

• CONCAVE

concave grating which (see DIFFRACTION OF LIGHT, § 8) allows us to dispense with all lenses, Rowland produced a revolution in spectroscopic measurement . At present we have still to content ourselves with a much diminished intensity of light when working with gratings, but there is some See also:

• HOPE, ANTHONY
• HOPE, THOMAS (c. 1770-1831)

hope that the efforts to concentrate the light into one spectrum will soon be successful . c . An See also:

• ECHELON (Fr. from ecltelle, ladder)

echelon grating . Imagine a See also:

• HORIZONTAL

horizontal See also:

• SECTION
• SECTION (Lat. sectio, cutting, secare, to cut)

section of a beam of light, and this section divided into a number of equal parts . Let somehow or other retardations be introduced so that the See also:

• OPTICAL

optical length of the successive parts increases by the same quantity nit, n being some number and the See also:

• WAVE

wave-length . If on emergence the different portions he brought together at the See also:

• FOCUS (Latin for " hearth " or " fireplace ")

focus it is obvious that the optical See also:

• ACTION

action must be in every respect similar to that of a grating when the nth order of spectrum is considered . A . Michelson produced the successive retardations by inserting step-by-step plates of See also:

• GLASS
• GLASS (O.E. glees, cf. Ger. Glas, perhaps derived from an old Teutonic root gla-, a variant of glo-, having the general sense of shining, cf. " glare," " glow ")
• GLASS, STAINED

glass of equal thickness so that the different portions of the beam traversed thicknesses of glass equal to nX, 2fX, 3nX, . . .

NA . The optical effect as regards resolving power is the same as with a grating of N lines in the nth order, but, nearly all the light not absorbed by the glass may be concentrated in one or two orders.' d . Some other appliance in which interference with See also:

• LONG, GEORGE (1800-1879)
• LONG, JOHN DAVIS (1838– )

long difference of path is made use of, such as the interferometer of Fabry and Perot, or Lummer's See also:

• PLATE

plate (see INTERFERENCE OF LIGHT) . The echelon and interferometer serve only a limited purpose, but must be called into action when the detailed structure of lines is to be examined . For the study of Zeeman effects (see MAGNETO-See also:

• OPTICS

OPTICS) the echelon seems specially adapted, while the great pliability of Fabry and Perot's methods, allowing a clear See also:

• INTERPRETATION (from Lat. interpretari, to expound, explain, inter pres, an agent, go-between, interpreter; inter, between, and the root pret-, possibly connected with that seen either in Greek 4 p4'ew, to speak, or irpa-rrecv, to do)

interpretation of results, is likely to secure them permanently an established See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place in measurements of precision . The power of a spectroscope to perform its main See also:

• FUNCTION

function, which is to See also:

• SEPARATE

separate vibrations of different but closely adjacent frequencies, is called its " resolving power." The See also:

• LIMITATION, STATUTES OF

limitation of power is introduced as in all optical instruments, by the finiteness of the length of a wave of light which causes the image of an indefinitely narrow slit to spread out over a finite width in the See also:

• FOCAL

focal See also:

• PLANE

plane of the observing telescope . The so-called " diffraction " image of a homogeneously illuminated slit shows a central See also:

• BAND

band limited on either See also:

• SIDE
• SIDE (mod. Eski Adalia)

side by a See also:

• LINE

line along which the ' Michelson, Astrophys . Journ . (1898), 8, p . 36; A . Schuster, Theory of Optics, p . 115.intensity is zero, and this band is accompanied by a number of fainter images corresponding to the diffraction of a See also:

• STAR

star image in a telescope .

See also:

• LORD
• LORD (O. Eng. hldford, i.e. hldfweard, the warder or keeper of bread, hlIf, loaf; the word is not represented in any other Teutonic language)
• LORD, JOHN (1810-1894)

Lord See also:

• RAYLEIGH, JOHN WILLIAM STRUTT

Rayleigh, to whom we owe the first See also:

• GENERAL
• GENERAL (Lat. generalis, of or relating to a genus, kind or class)

general discussion of the theory of the spectroscope, found by observation that if two spectroscopic lines of frequencies n, and n2 are observed in an instrument, they are just seen as two separate lines when the centre of the central diffraction band of one coincides with the first minimum intensity of the other . In that case the image of the See also:

• DOUBLE
• DOUBLE (from the Mid. Eng. duble, the form which gives the present pronunciation, through the Old Er. duble, from Lat. duplus, twice as much)

double line shows a diminution of intensity along the centre, just sufficient to give a clear impression that we are not dealing with a single line, and the intensity at the minimum is o•81 of that at the point of maximum See also:

• ILLUMINATION

illumination . We may say therefore that if the difference between the frequencies n, and n2 of the two waves is such that in the combined image of the slit the intensity at the minimum between the two See also:

• MAXIMA

maxima falls to o•81, the lines are just resolved and n,/(n,–n2) may then be called the resolving power . There is something arbitrary in this See also:

• DEFINITION (Lat. definitio, from de-finire, to set limits to, describe)

definition, but as the See also:

• PRACTICAL

practical importance of the question lies in the comparison between instruments of different types, the exact See also:

• STANDARD
• STANDARD, BATTLE OF THE

standard adopted is of See also:

• MINOR
• MINOR (Lat. for smaller, lesser)
• MINOR, ROBERT CRANNELL (1839-1904)

minor importance, the See also:

• CHIEF (from Fr. chef, head, Lat. caput)

chief consideration being simplicity of application . Lord Rayleigh's expression for the resolving power of different instruments is based on the See also:

• ASSUMPTION, FEAST OF

assumption that the geometrical image of the slit is narrow compared with the width of the diffraction image . This See also:

• CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition is necessary if the full power of the instrument is to be called into action . Unfortunately considerations of luminosity compel the observer often to widen the slit much beyond the range within which the theoretical value of resolving power holds in practice . The See also:

• EXTENSION (Lat. ex, out ; tendere, to stretch)

extension of the investigation to wide slits was first made by the present writer in the article " Spectroscopy " in the 9th edition of the See also:

• ENCYCLOPAEDIA

Encyclopaedia Britannica . Reconsideration of the subject led him afterwards to modify his views to some extent, and he has since more fully discussed the question.' Basing the investigation on the same criterion of resolution as in the case of narrow slits, we postulate for both narrow and wide slits that two lines are resolved when the intensity of the combined image falls to a value of o•810 in the centre between the lines, the intensity at the maxima being unity . We must now however introduce a new criterion the " purity " and distinguish it from the resolving power: the purity is defined by ni/(n,–n2), where n, and n2 are the frequencies of two lines such that they would just be resolved with the width of slit used . With an indefinitely narrow slit the purity is equal to the resolving power . As purity and resolving power are essentially See also:

• POSITIVE (or PORTABLE) ORGAN

positive quantities, n, in the above expression must be the greater of the two frequencies .

With wide slits the difference n,–n2 depends on their width . If we write P =p R where P denotes the purity and R the resolving power, we may See also:

• CALL (from Anglo-Saxon ceallian, a common Teutonic word, cf. Dutch kallen, to talk or chatter)

call p the " purity-See also:

• FACTOR (from Lat. facere, to make or do)

factor . " In the See also:

• PAPER
• PAPER (Fr. papier, from Lat. papyrus)

paper quoted the numerical values of p are given for different widths of slit, and a table shows to what extent the loss of purity due to a widening of the slit is accompanied by a gain in luminosity . The general results may be summarized as follows: if the width of the slit is equal to fA/4D (where X is the wave-length concerned, D the See also:

• DIAMETER (from the Cr. &a, through, µErpov, measure)

diameter of the collimator lens, and f its focal length) practically full resolving power is obtained and a further narrowing of the slit would See also:

• LEAD
• LEAD (pronounced iced)

lead to loss of light without corresponding gain . We call a slit of this width a " normal slit . " With a slit width equal to twice the normal one we lose 6% of resolution, but obtain twice the intensity of light . With a slit equal in width to eight times the normal one the purity is reduced to o•45R, so that we lose rather more than See also:

• HALF

half the resolving power and increase the light 3.7 times . If we widen the slit still further rapid loss of purity results, with very little gain in light, the maximum luminosity obtainable with an indefinitely wide slit being four times that obtained with the normal one . It follows that for observations in which light is a consideration spectroscopes should be used which give about twice the resolving power of that actually required; we may then use a slit having a width of nearly eight times that of the normal one . 2 Astrophys . Journ . (1905), 21, p .

197 . Theoretical resolving power can only be obtained when the whole collimator is filled with light and further (as pointed out by Lord Rayleigh in the course of discussion during a See also:

• MEETING (from " to meet," to come together, assemble, 0. Eng. metals ; cf. Du. moeten, Swed. mota, Goth. gamotjan, &c., derivatives of the Teut. word for a meeting, seen in O. Eng. Wit, moot, an assembly of the people; cf. witanagemot)

meeting of the " Optical See also:

• CONVENTION (Lat. conventio, an assembly or agreement, from convenire, to come together)
• CONVENTION, THE NATIONAL

Convention " in See also:

• LONDON

London, 1905) each portion of the collimator must be illuminated by each portion of the luminous source . These conditions may be generally satisfied by projecting the image of the source on the slit with a lens of sufficient See also:

• APERTURE (from Lat. aperire, to open)

aperture . When the slit is narrow light is lost through diffraction unless the angular aperture of this condensing lens, as viewed from the slit, is considerably greater than that of the collimator lens . When spectroscopes are used for stellar purposes further considerations have to be taken account of in their construction; and these are discussed in a paper by H . F . Newall.' 3 . Spectroscopic Measurements and See also:

• STANDARDS

Standards of Wave-Length.—All spectroscopic measurement should be reduced to wave-lengths or wave-frequencies, by a See also:

• PROCESS

process of See also:

• INTERPOLATION (from Lat. interpolare, to alter, or insert something fresh, connected with pollee, a polish)

interpolation between lines the wave-lengths of which are known with sufficient accuracy . The most convenient unit is that adopted by the See also:

• INTERNATIONAL, THE

International See also:

• UNION
• UNION (known locally as Union Hill and officially as Town of Union)

Union of Solar See also:

• RESEARCH (O. Fr. recerche, from recercher, re- and cercer, mod. chercher, to search; Late Lat. circare, to go round in a circle, to explore)

Research and is called an See also:

• ANGSTROM, ANDERS JONAS (1814-1874)

Angstrom (A) ; and is equal to 10 e See also:

• EMS

ems . A . Perot and C . Fabry, employing their interferometer methods, have compared the wave-length of the red See also:

• CADMIUM (symbol Cd, atomic weight 112.4 (0=16))

cadmium line with the standard See also:

• METRE ((terpuc?, sc. rFxvf, from Gr. Or poi', measure)

metre in See also:

• PARIS
• PARIS (also called ALEXANDROS)
• PARIS, ALEXIS PAULIN (1800-x881)
• PARIS, BRUNO PAULIN GASTON (1839-1903)
• PARIS, FRANCOIS DE (169o-1727)
• PARIS, LOUIS PHILIPPE ALBERT
• PARIS, TREATIES OF (1814-1815)

Paris and found it to be equal to 6438.4696 A, the observations being taken in dry See also:

• AIR (from an Indo-European root meaning " breathe," " blow ")
• AIR, or ASBEN

air at 18° C and at a pressure of 76 ems .

(g= 980.665) . This number agrees singularly well with that determined in 1893 by Michelson, who found for the same line 6438.4700 . Perot's number is now definitely adopted to define the Angstrom, and need never be altered, for should at some future See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time further researches reveal a See also:

• MINUTE
• MINUTE (Lat. minutes, small; minuere, to make less)

minute See also:

• ERROR (Lat. error, from errare, to wander, to err)

error, it will be only necessary to See also:

• CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change slightly the temperature or pressure of the air in which the wave-length is measured . A number of secondary standards separated by about 5o A, and See also:

• TERTIARY

tertiary standards at intervals of from 5 to 10 A have also been determined . By means of these, spectroscopists are enabled to measure by interpolation the wave-length of any line they may wish to determine . Interpolation is easy in the case of all observations taken with a grating . In the case of a prism some caution is necessary unless the standards used are very See also:

• CLOSE
• CLOSE (from Lat. clausum, shut)
• CLOSE, MAXWELL HENRY (1822-1903)

close together . The most convenient and accurate See also:

• FORMULA
• FORMULA (Lat. diminutive of forma, shape, pattern, &c., especially used of rules of judicial procedure)

formula of interpolation seems to be that discovered by J . F . See also:

• HARTMANN, KARL ROBERT EDUARD VON (1842–1906)
• HARTMANN, MORITZ (1821–1872)

Hartmann . If D is the measured deviation of a See also:

• RAY (Lat. raia)
• RAY (or WRAY, as he wrote his name till 1670), JOHN (1628-1705)

ray, and Do, X0, c and a are four constants, the See also:

• EQUATION (from Lat. aequatio, aequare, to equalize)

equation X=Xo+ (D —Do)uo seems to represent the connexion between deviation and wave-length with considerable accuracy for prisms constructed with the See also:

• ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary See also:

• MEDIA

media . The See also:

• CONSTANT, BENJAMIN (1845-1902)

constant a has the same value 1.2 for See also:

• CROWN

crown and See also:

• FLINT
• FLINT, AUSTIN (1812-1886)
• FLINT, or FLTNTSHTRE (sir Gallestr)
• FLINT, ROBERT (1838- )
• FLINT, TIMOTHY (1780-1840)

flint glass, so that there are only three disposable constants See also:

• LEFT

left .

In many cases it is sufficient to substitute unity for a and write _ ~o f t D —Do' which gives a convenient formula, which in this See also:

• FORM (Lat. forma)

form was first used by A . See also:

• CORNU, MARIE

Cornu . If within the range 5100-3700 A, the constants are determined once for all, the formula seems capable of giving by interpolation results accurate to o• 2 A, but as a rule the range to which the formula is applied will be much less with a corresponding gain in the accuracy of the results . Every observer should not only See also:

• RECORD (Lat. recordari, to recall to mind, from cor, heart or mind)

record the resolving power of the instrument he uses, but also the purity-factor as defined above . The resolving power in the case of gratings is simply mn, where m is the order of spectrum used, and il the total number of lines ruled on the grating . In the case of prisms the resolving power is—t (dµ/dX), where t is the effective thickness of the See also:

• MEDIUM

medium traversed by the ray . If 12 and t, are thicknesses traversed by the extreme rays, 1=t2—t,, and if, as is usually the case, the prism is filled right up to its See also:

• REFRACTION
• REFRACTION (Lat. refringere, to break open or apart)

refraction cap, t, = o, and becomes equal to the greatest thickness of the medium which is made use of . When compound prisms are used in which, ' Monthly Notices R.A.S . (1905), 65, p . 6o5 . for the purpose of obtaining smaller deviation, one See also:

• PART

part of the compound acts in opposition to the other, the resolving power of the opposing portion must be deducted in calculating the power of the whole . Opticians should See also:

• SUPPLY (through Fr. from Lat. supplere, to fill up)

supply sufficient See also:

• INFORMATION (from Lat. informare, to give shape or form to, to represent, describe)

information of the dispersive properties of their materials to allow dµ/dX to be calculated easily for different parts of the spectrum .

The determination of the purity-factor requires the measurement of the width of'the slit . This is best obtained by optical means . The collimator of a spectroscope should be detached, or moved so as to admit of the introduction of an See also:

• AUXILIARY (from Lat. auxilium, help)

auxiliary slit at a distance from the collimator lens equal to its focal length . If a source of light be placed behind the auxiliary slit a parallel beam of light will pass within the collimator and fall on the slit the width of which is to be measured . With fairly homogeneous light the diffraction See also:

• PATTERN

pattern may be observed at a distance, varying with the width of the slit from about the length of the collimator to one See also:

• QUARTER (through Fr. from Lat. quartarius, fourth part)

quarter of that length . From the measured distances of the diffraction bands the width of the slit may be easily deduced . 4 . Methods of Observation and Range of Wave-Lengths.—Visual observation is limited to the range of frequencies to which our eyes are sensitive . Defining oscillation as is usual in spectroscopic measurement by wave-length, the visible spectrum is found to extend from about 7700 to 3900 A . In importance next to visual observation, and in the See also:

• OPINION (Lat. opinio, from opinari, to think)

opinion of some, surpassing it, is the photographic method . We are enabled by means of it to extend materially the range of our observation, especially if the ordinary kinds of glass, which strongly absorb ultra-See also:

• VIOLET

violet light, are avoided, and, when necessary, replaced by See also:

• QUARTZ

quartz . It is in this manner easy to reach a wave-length of 3000 A, and, with certain precautions, 1800 A .

At that point, however, quartz and even atmospheric air become strongly absorbent and the expensive fluorspar becomes the only medium that can be used . See also:

• HYDROGEN [symbol H, atomic weight x-oo8 (0=16)]

Hydrogen still remains transparent . The beautiful researches of V . See also:

• SCHUMANN, ROBERT ALEXANDER (1810-1856)

Schumann 2 have shown, however, that with the help of spectroscopes void of air and specially prepared photographic plates, spectra can be registered as far down as 1200 A . Lyman more recently has been able to obtain photographs as far down as 1030 A with the help of a concave grating placed in vacuo.3 Although the vibrations in the infra-red have a considerably greater intensity, they are more difficult to See also:

• REGISTER

register than those in the ultra-violet . Photographic methods have been employed successfully by See also:

• SIR

Sir W . Abney as far as 20,000 A, but long exposures arc necessary . Bolo-metric methods may be used with facility and advantage in the investigation of the distribution of intensities in continuous or semi-continuous spectra but difficulties are met with in the case of line spectra . See also:

• GOOD, JOHN MASON (1764-1827)

Good results in this respect have been obtained by B . W . Snow4 and by E . P .

See also:

• LEWIS, HENRY CARVILL
• LEWIS, JOHN FREDERICK (180 1876)
• LEWIS, MATTHEW GREGORY (1775-1818)
• LEWIS, MERIWETHER (1774-1809)
• LEWIS, SIR GEORGE CORNEWALL, BART

Lewis,' lines as far as 11,500 having been measured by the latter . More recently F . Paschen 6 has further extended the method and added a number of infra-red lines to the spectra of See also:

• HELIUM (from Gr. i)Xcor, the sun)

helium, See also:

• ARGON (from the Gr. 6.-, privative, and Epyov, work; hence meaning " inert ")

argon, See also:

• OXYGEN (symbol 0, atomic weight 16)

oxygen and other elements . In the case of helium one line was found with a wave-length of 20,582 A . C . V . Boys' microradiometer has occasionally been made use of, and the extreme sensitiveness of the See also:

• CROOKES, SIR WILLIAM (1832– )

Crookes' See also:

• RADIOMETER

radiometer has also given excellent results in the hands of H . See also:

• RUBENS, PETER PAUL (1577-164o)

Rubens and E . F . See also:

• NICHOLS, JOHN (1745–1826)

Nichols . In the opinion of the writer the latter instrument will ultimately replace the bolometer, its only disadvantage being that the radiations have to See also:

• TRAVERSE

traverse the side of a See also:

• VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

vessel, and are therefore • subject to absorption . In order to record line spectra it is by no means necessary that the receiving instrument (bolometer or radio-See also:

• METER, ELECTRIC

meter) should be linear in shape, for the separation of adjacent lines may be obtained if the linear See also:

• RECEIVER

receiver be replaced by a narrow slit in a See also:

• SCREEN (usually, but very doubtfully, connected with Lat. scrinium, a box for holding books, from scribere, to write; a connexion with Ger. Schranke, barrier, has been suggested)

screen placed at the focus of the condensing lens .

The sensitive See also:

• VANE (formerly spelt " fane," i.e. pennon, flag; cf. Ger. Feline, Du. vaan, Fr. girouette, Ital. banderuola, Ger. Wellerfahne)
• VANE, SIR HENRY (1589-1654)
• VANE, SIR HENRY (1613-1662)

vane or See also:

• STRIP

strip may then be placed behind the slit; its width will not affect the resolving power though there may be a diminution of sensitiveness . The longest waves Wied . Annalen (1901), 5, p . 349 . Astrophys . Journ . (1906), 23, p . 181 . 4 Wied . Annalen (1892), 47, p . 208 . Astrophys .

_Town . (1895), 2, p . I . e Drude Annalen (1908), 27, p . 537 and (1909), 29 . not necessary, in fact better results are obtained without it . Lecoq de Boisbaudran has applied this method with considerable success, and it is to be recommended whenever only small electric power is at the disposal of the observer . To diminish the resistance the current should pass through as small a layer of liquid as possible . It is convenient to place the liquid in a See also:

• SHORT, FRANCIS JOB (1857– )

short See also:

• TUBE (Lat. tuba)

tube, a See also:

• PLATINUM [symbol Pt, atomic weight 145.0 (0=16)]

platinum See also:

• WIRE (A.S. wir, a wire; cf. Swed. vire, to twist, M.H.G. wiere, a gold ornament, Lat. viriae, armlets, ultimately from the root wi, to twist, bind)

wire sealed in at the bottom to convey the current reaching to the level of the open end . If a thick-walled capillary tube is passed over the platinum tube and its length so adjusted that the liquid rises in it by capillary action just above the level of the tube, the spectrum may be examined directly, and the loss of light due to the passage through the partially wetted See also:

• SURFACE

surface of the walls of the tube is avoided . For the investigation of the spectra of gases at reduced pressures the so-called See also:

• PLUCKER, JULIUS (18or-1868)

Plucker tubes (more generally but incorrectly called See also:

• GEISSLER, HEINRICH (1814-1879)

Geissler tubes) are in See also:

• COMMON

common use . When the pressure becomes very See also:

• LOW, SETH (1850- )
• LOW, WILL HICOK (1853- )

low, inconvenience arises owing to the difficulty of establishing the See also:

• DISCHARGE (adapted from the O. Fr. discharge, modern decharge, from a med. Lat. discargare, to unload, dis- and earn care, to load, cf. " charge ")

discharge .

In that case the method introduced by J . J . See also:

• THOMSON, JAMES (1822-1892)
• THOMSON, JAMES (1834-1882)
• THOMSON, JAMES (r700-1748)
• THOMSON, JOHN (1778-184o)
• THOMSON, JOSEPH (1858-1895)
• THOMSON, SIR CHARLES WYVILLE (1830-1882)
• THOMSON, THOMAS (1773-1852)
• THOMSON, WILLIAM (1819-189o)

Thomson might with advantage be more frequently employed . Thomson e places spherical bulbs inside thick See also:

• SPIRAL

spiral conductors through which the oscillating discharge of a powerful See also:

• BATTERY (Fr. batterie, from battles, to beat)

battery is led . The rapid variation in the intensity of the magnetic See also:

• FIELD (a word common to many West German languages, cf. Ger. Feld, Dutch veld, possibly cognate with O.E. f olde, the earth, and ultimately with root of the Gr. irAaror, broad)
• FIELD, CYRUS WEST (1819-1892)
• FIELD, DAVID DUDLEY (18o5-1894)
• FIELD, EUGENE (1850-1895)
• FIELD, FREDERICK (18o1—1885)
• FIELD, HENRY MARTYN (1822-1907)
• FIELD, JOHN (1782—1837)
• FIELD, MARSHALL (183 1906)
• FIELD, NATHAN (1587—1633)
• FIELD, STEPHEN JOHNSON (1816-1899)
• FIELD, WILLIAM VENTRIS FIELD, BARON (1813-1907)

field causes a brilliant electrodeless discharge which is seen in the form of a See also:

• RING (O.E. hring; a word common to Teutonic languages; and probably cognate with the Lat. circus, Gr. KtpKOS or KpLKOS, Skt'. chakra, wheel, circle, cf. also " harangue "); in art, a band of circular shape of varying sizes, made of any material and used f

ring passing near the inner walls of the bulb when the pressure is properly adjusted . A variety of methods to render gases luminous should be at the command of the investigator, for nearly all show some distinctive peculiarity and any new modification generally results in fresh facts being brought to light . Thus E . Goldstein' was able to show that an increase in the current See also:

• DENSITY (Lat. densus, thick)

density is capable of destroying the well-known spectra of the See also:

• ALKALI

alkali metals, replacing them by quite a new set of lines . 6 . Theory of See also:

• RADIATION, THEORY OF

Radiation.—The general recognition of spectrum analysis as a method of physical and chemical research occurred simultaneously with the theoretical See also:

• FOUNDATION (Lat. fundatio, from fundare, to found)

foundation of the connexion between radiation and absorption . Though the experimental and theoretical developments were not necessarily dependent on each other, and by far the larger proportion of the subject which we now See also:

• TERM

term " Spectroscopy " could stand irrespective of Gustav See also:

• KIRCHHOFF, GUSTAV ROBERT (1824-1887)
• KIRCHHOFF, JOHANN WILHELM ADOLF (1826-1908)

Kirchhoff's thermodynamical investigations, there is no doubt that the latter was, historically speaking, the immediate cause of the feeling of confidence with which the new branch of science was received, for nothing impresses the scientific world more strongly than just that little See also:

• TOUCH (derived through Fr. toucher from a common Teutonic and Indo-Germanic root, cf. " tug," " tuck," O. H. Ger. zucchen, to twitch or draw)

touch of See also:

• MYSTERY (Gr. µw(Trilpcov, from tcuvrm, an initiate, µfew, to shut the mouth)

mystery which attaches to a mathematical investigation which can only be understood by the few, and is taken on See also:

• TRUST

trust by the many, provided that the author is a See also:

• MAN
• MAN, ISLE OF (anc. Mona)

man who commands general confidence . While See also:

• BALFOUR, FRANCIS MAITLAND (1851-1882)
• BALFOUR, ROBERT (known also as BALFOREUS) (1550?-1625?)
• BALFOUR, SIR JAMES (of Pittendreich) (d. 1583 Or 1584)
• BALFOUR, SIR JAMES, BART

Balfour See also:

• STEWART, ALEXANDER TURNEY (1803-1876)
• STEWART, BALFOUR (1828-1887)
• STEWART, CHARLES (1778–1869)
• STEWART, DUGALD (1753-1828)
• STEWART, J
• STEWART, JOHN (1749—1822)
• STEWART, JULIUS L
• STEWART, SIR DONALD MARTIN (1824–19o0)
• STEWART, SIR HERBERT (1843—1885)
• STEWART, SIR WILLIAM (c. 1540—c. 1605)
• STEWART, STUART
• STEWART, WILLIAM (c. 1480-c. 1550)

Stewart's See also:

• WORK

work on the theory of exchanges was too easily understood and therefore too easily ignored, the weak points in Kirchhoff's developments are only now beginning to be perceived .

The investigations both of Balfour Stewart and of Kirchhoff are based on the See also:

• IDEA (Gr. Ibia, connected with i&eiv, to see; cf. Lat. species from specere, to look at)

idea of an enclosure at See also:

• UNIFORM

uniform temperature and the general results of the reasoning centre in the conclusion that the introduction of any See also:

• BODY

body at the same temperature as the enclosure can make no difference to the streams of radiant See also:

• ENERGY (from the Gr. ivipyela; iv, in, ipyov, work)

energy which we imagine to traverse the enclosure . This result, which, accepting the possibility of having an absolutely opaque enclosure of uniform temperature, was clearly proved by Balfour Stewart for the total radiation, was further extended by Kirchhoff, who applied it (though not with mathematical rigidity as is sometimes supposed) to the separate wave-lengths . All Kirchhoff's further conclu- sions are based on the assumption that the radiation transmitted through a partially transparent body can be expressed in terms of two See also:

• INDEPENDENT

independent factors (1) an absorption of the incident radiation, and (2) the radiation of the absorbing medium, which See also:

• SOLUTION (from Lat. solvere, to loosen, dissolve)

solution the spark may be taken from the solution, which must takes place equally in all directions . It is assumed further that the absorption is proportional to the incident radiation and (at any See also:

• RATE

rate approximately) independent of the temperature, while the radiation is assumed to be a function of the temperature 8 Phil . Mag . 32, pp . 321, 445 . Vertr. d. plays . Ges . (1904), 9, p . 321 .. observed up to the present are those recorded by H .

Rubens and E . Aschkinass' (•oo61 ems. or 610,000 A) . 5 . Methods of Rendering Gases Luminous.—The extreme flexibility of the phenomena shown by radiating gases renders it a See also:

• MATTER

matter of great importance to examine them under all possible conditions of luminosity . Gases, like atmospheric air, hydrogen or See also:

• CARBON (symbol C, atomic weight 12)

carbon dioxide do not become luminous if they are placed in tubes, even when heated up far beyond See also:

• WHITE
• WHITE, ANDREW DICKSON (1832– )
• WHITE, GILBERT (1720–1793)
• WHITE, HENRY KIRKE (1785-1806)
• WHITE, HUGH LAWSON (1773-1840)
• WHITE, JOSEPH BLANCO (1775-1841)
• WHITE, RICHARD GRANT (1822-1885)
• WHITE, ROBERT (1645-1704)
• WHITE, SIR GEORGE STUART (1835– )
• WHITE, SIR THOMAS (1492-1567)
• WHITE, SIR WILLIAM ARTHUR (1824--1891)
• WHITE, SIR WILLIAM HENRY (1845– )
• WHITE, THOMAS (1628-1698)
• WHITE, THOMAS (c. 1550-1624)

white See also:

• HEAT (0. E. haktu, which like " hot," Old Eng. hat, is from the Teutonic type haita, hit, to be hot; cf. Ger. hitze, heiss; Dutch, hitte, heet, &c.)

heat as in the electric See also:

• FURNACE

furnace . This need not necessarily be interpreted as indicating the impossibility of rendering gases luminous by temperature only, for the transparency of the See also:

• GAS

gas for luminous radiations may be such that the emission is too weak to be detected . When there is appreciable absorption as in the case of the vapours of See also:

• CHLORINE (symbol Cl, atomic weight 35`46 (0=16)

chlorine, See also:

• BROMINE (symbol Br, atomic weight 79-96)

bromine, See also:

• IODINE (symbol I, atomic weight '26.92)

iodine, See also:

• SULPHUR

sulphur, See also:

• SELENIUM [symbol Se, atomic weight 79.2 (O=16)]

selenium and See also:

• ARSENIC (symbol As, atomic weight 75.0)

arsenic, luminosity begins at a red heat . Thus G . Salet2 observed that iodine gives a spectrum of See also:

• BRIGHT, JOHN (1811-188g)
• BRIGHT, SIR CHARLES TILSTON

bright bands when in contact with a platinum spiral made white hot by an electric current, and J . Evershed3 has shown that in this and other cases the temperature at which emission becomes appreciable is about 700° . It is only recently that owing to the introduction of carbon tubes heated electrically the excitement of the luminous vibrations of molecules by temperature alone has become an effective method for the study of their spectra even in the case of metals . Hitherto we were entirely and still are generally confined to See also:

• ELECTRICAL
• ELECTRICAL (or ELECTROSTATIC) MACHINE

electrical excitation or to chemical action as in the case of flames .

In the ordinary laboratory the See also:

• BUNSEN, CHRISTIAN CHARLES JOSIAS, BARON VON (1791-186o,)
• BUNSEN, ROBERT WILHELM VON (1811-1899)

Bunsen See also:

• FLAME (Lat. flamma; the root flag- appears in flagrare, to burn, blaze, and Gr. d, XEryew)

flame has become universal, and a number of substances, such as the salts of the alkalis and alkaline earths, show characteristic spectra when suitably placed in it . More information may. be gained with the help of the oxyhydrogen flame, which with its higher temperature has not been used as frequently as it might have been, but W . N . See also:

• HARTLEY, DAVID (1705–1757)
• HARTLEY, JONATHAN SCOTT (1845– )
• HARTLEY, SIR CHARLES AUGUSTUS (1825– )

Hartley has employed it with great success, and in See also:

• CYANITE

cyanite (a silicate of See also:

• ALUMINIUM (symbol Al; atomic weight 27.0)

aluminium) has found a material which is infusible at the temperature of this flame, and is therefore suitable to hold the substance which it is desired to examine . An interesting and instructive manner of introducing salts into flames was discovered by A . Gouy, who forced the air before it entered the Bunsen burner, through a spray produce containing a See also:

• SALT (a common Teutonic word, cf. Dutch zout, Ger. Salz, Scand. salt; cognate with Gr. &Xs, Lat. sal)
• SALT, SIR TITUS, BART

salt in solution . By this method even such metals as See also:

• IRON [symbol Fe, atomic weight 55.85 (0=16)]

iron and See also:

• COPPER (symbol Cu, atomic weight 63.1, H=1, or 63.6, O = 16)

copper may be made to show some of their characteristic lines in the Bunsen burner . The spectra produced under these circumstances have been studied in detail by C. de Watteville.° Of more frequent. use have been electric methods, owing to the greater intensity of the radiations which they yield . Especially when large gratings are employed do we find that the electric arc alone seems sufficient to give vibrations of the requisite power . The metals may be introduced into the arc in various ways, and in some cases where they can be obtained in sufficient quantity the metallic electrodes may be used in the place of carbon poles . The usual method of obtaining spectra by the discharges from a Ruhmkorff coil or Wimshurst See also:

• MACHINE
• MACHINE (through Fr. from Lat. form machina of Gr. µnxavil)

machine needs no description . The effects may be varied by altering the capacity and self-See also:

• INDUCTION (from Lat. inducere, to lead into; cf. Gr. ilrayu yli)

induction of the See also:

• CIRCUIT (Lat. circuitus, from circum, round, and ire, to go)

circuit which contains the spark See also:

• GAP

gap .

The insertion of self-induction has the advantage of avoiding the lines due to the gas through which the spark is taken, but it introduces otber changes in the nature of the spark, so that the results obtained with and without self-induction are not directly comparable . See also:

• COUNT (Lat. comes, gen. comitis, Fr. comte, Ital. conte, Span. conde)

Count See also:

• GRAMONT, ANTOINE AGENOR ALFRED
• GRAMONT, PHILIBERT, COMTE DE (1621-1707)

Gramont 5 has been able to obtain spectroscopic See also:

• EVIDENCE (Lat. evidentia, evideri, to appear clearly)

evidence of the metalloids in a See also:

• MINERAL

mineral by employing powerful condensers and See also:

• HEATING

heating the electrodes in an oxyhydrogen flame when these (as is often the case) are not sufficiently conducting . When the substance to be examined spectroscopically is in then be used as kathode of air . The See also:

• CONDENSER

condenser is in this case ' Wied . Annalen (1898), 65, p . 241 . ' See also:

• ANN

Ann . Chim . Phys . (1873), 28 . ' Phil . Mag .

(1895), 39, p . 460 . • Phil . Trans . (1904), 204, A. p . 139 . Comptes rendus, vols . 121, 122, 124 . only and independent of the temperature of the enclosure . This See also:

• DIVISION (from Lat. dividere, to break up into parts, separate)

division into absorption and radiation is to some extent artificial and will have to be revised when the ohenomena of radiation are placed on a See also:

• MECHANICAL

mechanical basis . For our present purpose it is only necessary to point out the difficulty involved in the assumption that the radiation of a body is independent of the temperature of the enclosure . The present writer See also:

• DREW
• DREW, SAMUEL (1765-1833)

drew attention to this difficulty as far back as 1881,1 when he pointed out that the different intensities of different spectral lines need not involve the consequence that in an enclosure of uniform temperature the energy is unequally partitioned between the corresponding degrees of freedom .

When the See also:

• MOLECULE (from mod. Lat. molecula, the diminutive of moles, a mass)

molecule is losing energy the intensity of each See also:

• KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

kind of radiation depends principally on the rapidity with which it can be renewed by molecular impacts . The unequal intensities observed indicate a difference in the effectiveness of the channels through which energy is lost, and this need not be connected with the ultimate state of See also:

• EQUILIBRIUM (from the Lat. aequus, equal, and libra, a balance)

equilibrium when the body is kept at a uniform temperature . For our immediate purpose these considerations are of importance inasmuch as they See also:

• BEAR
• BEAR, BLACK
• BEAR, BROWN
• BEAR, GRIZZLY
• BEAR, ISABELLINE
• BEAR, WHITE

bear on the question how far the spectra emitted by gases are thermal effects only . We generally observe spectra under conditions in which dissipation of energy takes place, and it is not obvious that we possess a definition of temperature which is strictly applicable to these cases . When, for instance, we observe the relation of the gas contained in a Plucker tube through which an electric discharge is passing, there can be little doubt that the See also:

• PARTITION

partition of energy is very different from what it would be in thermal equilibrium . In consequence the question as to the connexion of the spectrum with the temperature of the gas seems to the present writer to lose some of its force . We might define temperature in the case of a flame or vacuum tube by the temperature which a small totally reflecting body would tend to take up if placed at the spot, but this definition would fail in the case of a spark discharge . Adopting the definition we should have no difficulty in proving that in a vacuum tube gases may be luminous at very low temperatures, but we are doubtful whether such a conclusion is very helpful towards the elucidation of our problem . Radiation is a molecular process, and we can speak of the radiation of a molecule but not of its temperature . When we are trying to bring radiation into connexion with temperature, we must therefore take a sufficiently large See also:

• GROUP

group of molecules and compare their See also:

• AVERAGE

average energies with the average radiation . The question arises whether in a vacuum discharge, in which only a comparatively small proportion of the molecules are affected, we are to take the average radiation of the affected portion or include the whole See also:

• LOT
• LOT (Lat. Oltis)

lot of molecules, which at any moment are not concerned in the discharge at all . !i'he two processes would lead to entirely different results .

The problem, which, in the opinion of the present writer, is the one of See also:

• INTEREST

interest and has more or less definitely been in the minds of those who have discussed the subject, is whether the type of wave sent out by a molecule only depends on the See also:

• INTERNAL

internal energy of that molecule, or on other considerations such as the mode of excitement . The average energy of a medium containing a mixture of dissimilar elements possesses in this respect only a very secondary interest . We must now inquire a little more closely into the mechanical conception of radiation . According to present ideas, the wave originates in a disturbance of electrons within the molecules . The electrons responsible for the radiation are probably few and not directly involved in the structure of the See also:

• ATOM (Gr. &rows, indivisible, from a- privative, and TE,aPfw, to cut)

atom, which according to the view at present in favour, is itself made up of electrons . As there is undoubtedly a connexion between thermal See also:

• MOTION (Lat. motio, from movere, to move)
• MOTION, LAWS OF

motion and radiation, the energy of these _electrons within the atom must be supposed to increase with temperature . But we know also .that in the complete radiation of a white body the radiative energy increases with the See also:

• FOURTH

fourth power of the See also:

• ABSOLUTE (Lat. absolvere, to loose, set free)

absolute temperature . Hence a part of what must be included in thermal energy is not simply proportional to temperature as is commonly assumed . The energy of radiation resides in the medium and not in the molecule . Even at the 1 Phil . Mag . (1881), 12, p .

261.highest temperatures at our command it is small compared with the energy of translatory motion, but as the temperature increases, it must ultimately gain the upper See also:

• HAND
• HAND (a word common to Teutonic languages; cf. Ger. Hand, Goth. handus)
• HAND, FERDINAND GOTTHELF (1786-185r)

hand, and if there is anywhere such a temperature as that of several million degrees, the greater part of the total energy of a body will be outside the atom and molecular motion ultimately becomes negligible compared with it . But these speculations, interesting and important as they are, lead us away from our main subject . Considering the great variety of spectra, which one and the same body may possess, the idea lies near that free electrons may temporarily attach themselves to a molecule or detach themselves from it, thereby altering the constitution of the vibrating See also:

• SYSTEM

system . This is most likely to occur in a discharge through a vacuum tube and it is just there that the greatest variety of spectra is observed . It has been denied by some that pure thermal motion can ever give rise to line spectra, but that either chemical action or impact of electrons is necessary to excite the See also:

• REGULAR

regular oscillations which give rise to line spectra . There is no doubt that the impact of electrons is likely to be effective in this respect, but it must be remembered that all bodies raised to a sufficient temperature are found to eject electrons, so that the presence of the free electrons is itself a consequence of temperature . The view that visible radiation must be excited by the impact of such an See also:

• ELECTRON

electron is therefore quite consistent with the view that there is no essential difference between the excitement due to chemical or electrical action and that resulting from a sufficient increase of temperature . Chemical action has frequently been suggested as being a necessary factor in the luminosity of flame, not only in the sense that it causes a sufficient rise of temperature but as furnishing some See also:

• SPECIAL

special and See also:

• PECULIAR

peculiar though undefined stimulus . An important experiment by C . See also:

• GUNTHER, JOHANN CHRISTIAN (1695-1723)

Gunther 2 seems however to show that the radiation of metallic salts in a flame has an intensity equal to that belonging to it in virtue of its temperature . • If a short length of platinum wire be inserted vertically into a lighted Bunsen burner the luminous line may be used as a slit and viewed directly through a prism . When now a small See also:

• BEAD

bead of a salt of See also:

• SODIUM [symbol Na, from Lat. natrium; atomic weight 23.00 (0=16)]

sodium or See also:

• LITHIUM [symbol Li, atomic weight 7•oo (0=16)]

lithium is placed in the flame the spectrum of the white hot platinum is traversed by the dark absorption of the D lines .

This is consistent with Kirchhoff's See also:

• LAW
• LAW (0. Eng. lagu, M. Eng. lawe; from an old Teutonic root lag, " lie," what lies fixed or evenly; cf. Lat. lex, Fr. loi)
• LAW, JOHN (1671—1729)
• LAW, WILLIAM (1686-1761)

law and shows that the sodium in a flame possesses the same relative radiation and absorption as sodium vapour heated thermally to the temperature of the flames . According to independent experiments by Paschen the radiation of the D line sent out by the sodium flame of sufficient density is nearly equal to that of a See also:

• BLACK
• BLACK, ADAM (1784-1874)
• BLACK, JEREMIAH SULLIVAN (1810–1883)
• BLACK, WILLIAM (1841-1898)

black body at the same temperature . 3 Other more See also:

• RECENT

recent experiments confirm the idea that the radiation of flames is mainly determined by their temperature . The definition of temperature given above, though difficult in the case of a flame and perhaps still admissible in the case of an electric arc, becomes See also:

• PRECARIOUS

precarious when applied to the disruptive phenomena of a spark discharge . The only sense in which we might be justified in using the word temperature here is by taking account of the energy set free in each discharge and distributing it between the amount of matter to which the energy is supplied . With a guess at the specific heat we might then calculate the maximum temperature to which the substance might be raised, if there were no loss by radiation or otherwise . But the molecules affected by a spark discharge are not in any sense in equilibrium as regards their partition of energy and the word " temperature " cannot therefore be applied to them in the ordinary sense . We might probably with advantage find some definition of what may be called " radiation temperature " based on the relation between radiation and absorption in Kirchhoff's sense, but further information based on experimental investigation is required . 7 . Limits of Homogeneity and Structure of Lines.—As a first approximation we may say that gases send out homogeneous 2 Wied . Ann . (1877), 2, p .

477 . Ibid . (1894), 51, p . 40 . radiations . A homogeneous oscillation is one which for all time is described by a circular function such as See also:

• SIN
• SIN (O. Eng. syn: a common Teutonic word, cf. Dutch zonde, Ger. Siinde)

sin(nt+a), t being the time and n and a constants . The qualification that the circular function must apply to all time is important, and unless it is recognized as a necessary condition of homogeneity, confusion in the more intricate problems or radiation becomes inevitable . Thus if a molecule were set into vibration at a specified time and oscillated according to the above equation during a finite See also:

• PERIOD
• PERIOD (Gr. irepioSos, a going or way round, circuit, aepi, round, and &Sis, way, road)

period, it would not send out homogeneous vibrations . In interpreting the phenomena observed in a spectroscope, it is necessary to remember that the instrument, as pointed out by Lord Rayleigh, is itself a producer of homogeneity within the limits defined by its resolving power . A spectroscope may be compared to a mechanical See also:

• HARMONIC

harmonic analyser which when fed with an irregular function of one variable represented by a See also:

• CURVE (Lat. curvus, bent)

curve supplies us with the sine curves into which the See also:

• ORIGINAL

original function may be resolved . This See also:

• ANALOGY (Gr. avaXo-yLa, proportion)

analogy is useful because the application of See also:

• FOURIER, F
• FOURIER, FRANCOIS CHARLES MARIE (1772-1837)
• FOURIER, JEAN BAPTISTE JOSEPH (1768-1830)

Fourier's analysis to the optical theory of spectroscopes has been doubted, and it may be urged in See also:

• ANSWER (derived from and, against, and the same root as swear)

answer to the objections raised that the instrument acts in all respects like a mechanical analyser,' the applicability of which has never been called into question . A limit to homogeneity of radiation is ultimately set by the so-called Doppler effect, which is the change of wave-length due to the translatory motion of the vibrating molecule from or towards the observer .

If N be the frequency of a homogeneous vibration sent out by a molecule at See also:

• REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

rest, the apparent frequency will be N v/ V), where V is the velocity of light and v is the velocity of the line of sight, taken as positive if the distance from the observer increases . If all molecules moved with the velocity of mean square, the line would be See also:

• DRAWN

drawn out into a band having on the frequency See also:

• SCALE
• SCALE (1) A

scale a width 2Nv/V, where v is now the velocity of mean square . According to See also:

• MAXWELL
• MAXWELL, JAMES CLERK (1831–1879)

Maxwell's law, however, the number of molecules having a velocity in the line of sight lying between v and v+dv is proportional to e—Rmdv, where /3 is equal to 3/2u2; for v=u, we have therefore the ratio in the number of molecules having velocity u to those having no velocity in the line of sight e—See also:

• RUE