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direct observation of the sources themselves does not yield satisfactory results . It is found to be much better to compare the illuminations produced on dead See also:

white surfaces from which no See also:

REGULAR

regular reflection takes See also:

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

place, or through colourless translucent material uniformly illuminated by the light placed on the further See also:

side . By such processes there is always loss of light, and we must be certain that the various coloured constituents of the light are reduced in the same proportion . This necessary See also:

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

condition is practically satisfied by the use of white diffusing screens . Two principles of See also:

RADIATION, THEORY OF

radiation underlie many photometric applications, namely, the inverse square distance See also:

law, and J . H . See also:

Lambert's " cosine law." Both can be established /averse on theoretical grounds, certain conditions being Square fulfilled .

But as these conditions are never abso- Distance lutely satisfied, the applicability of the two See also:

LAWS

laws Law. must in the end be tested by experiment . Since we find that within the errors of observation four candles, placed together at a distance of 2 ft. from a diffusing 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, produce the same illumination as one candle at a distance of 1 ft., we may regard the inverse square distance law as satisfied . Thus if two lights of intensities A and B produce equal illuminations on a screen when their distances from the screen are respectively a and b, we at once write down the relation between the two intensities in the See also:

FORM (Lat. forma)

form A : B=See also:

A2(z)

a2 : b2 . The theoretical basis of the law follows at once from the universally accepted view that light is See also:

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

energy radiating outwards in all directions from the source . If we assume that there is no loss of energy in the transmitting See also:

MEDIUM

medium, then the whole amount of radiant energy passing in one second across any closed surface completely surrounding the source of light 'must be the same whatever the size or form of the surface . Imagine for simplicity a point source of light, or its See also:

EQUIVALENT

equivalent, a uniformly radiating spherical surface with the point at its centre, and draw See also:

ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

round this point a spherical surface of unit See also:

RADIUS

radius . Across this surface there will pass a definite amount of radiant energy, in other words a definite See also:

TOTAL

total luminous See also:

FLUX (Lat. fluxus, a flowing; this being also the meaning of the English term in medicine, &c.)

flux, E, which will be the same for all concentric spherical surfaces . Since the See also:

AREA

area of a spherical surface of radius r is 41r See also:

R2(rt-s2)

r2, the flux which crosses unit area is E/4 7r r2 . This quantity is the " illumination." It is measured in terms of the unit called. the lux, which is defined as the illumination produced by a light of unit intensity on a perfectly white surface at a distance of 1 ft . In the great See also:

MAJORITY (Fr. majorite; Med. Lat. majoritas; Lat. major, greater)

majority of photometers the illuminations are compared, and the intensities are deduced by applying the law of the squared distances . Lambert's cosine law has to do with the way in which a luminous surface sends off its radiations in various directions . It is a See also:

MATTER

matter of See also:

COMMON

common observation that the disk of the See also:

sun appears equally See also:

bright all over the surface .

Careful measurements show that this is not strictly true; but it is sufficiently near the truth to suggest that under certain definable conditions the law 1 would hold accurately . Again, when a glowing surface is viewed through a small hole in an opaque See also:

PLATE

plate, the brightness is very approximately See also:

INDEPENDENT

independent of the angular position of the incandescent surface . This is the same phenomenon as the first mentioned, and shows that the more oblique, and therefore larger, See also:

ELEMENT (Lat. elementum)

element of surface sends the same amount of radiation through the hole . Hence the amount per unit surface sent off Lambert's Cosine Law . at a given See also:

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle with the normal must be less than that sent off in the direction of the normal in the inverse ratio of the areas of the corresponding normal and oblique elements; that is, as the cosine of the given angle to unity . For most practical purposes, and so See also:

long as the obliquity is not great, Lambert's law may be assumed to hold . In almost all accurate methods of photometry the aim is to bring the illuminating powers of the two sources to equality . This may be effected by altering the distance of either light from the illuminated surface . Or we may use polarized light and diminish the intensity of the stronger See also:

beam by suitable rotation of a See also:

Nicol See also:

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

prism, a method particularly useful in spectrophotometers . The same result may also be effected by inter-posing absorbent disks, the precise absorbing powers of which must, however, be known with great accuracy . Another useful method is that first described by H . See also:

Fox See also:

Talbot in 1834, and used with effect by See also:

PROFESSOR (the Latin noun formed from the verb profiteri, to declare publicly, to acknowledge, profess)

Professor See also:

William See also:

Swan (1849), and more recently by See also:

Sir W. de W .

Abney . Talbot's law is thus enunciated by H. von See also:

HELMHOLTZ, HERMANN LUDWIG FERDINAND VON (1821-1894)

Helmholtz: " When any See also:

PART

part of the retina is excited by regularly periodic intermittent light, and when the See also:

period is sufficiently See also:

SHORT, FRANCIS JOB (1857– )

short, the resulting impression will be continuous, and will be the same as that which would be produced if the whole light were distributed uniformly throughout the whole period." Talbot deduced the principle from the well-known experiment in which a continuous luminous See also:

LINE

line is produced by rapid rotation of a luminous point . If the principle be granted, it is obvious that any mechanism by which a See also:

ray of light is obstructed in a regularly rhythmic manner during definite intervals t', separated by intervals t, during which the light is allowed to pass, will have the effect of reducing the apparent brightness of the ray in the ratio t/(t + t') . This is frequently accomplished by placing in the ray a rotating disk perforated by radial sectors, the so-called Talbot disk . If photometric results are to be of See also:

general value it is essential to have a unit in which to See also:

EXPRESS (through the French from the past participle of the Lat. exprimere, to press out, by transference used of representing objects in painting or sculpture, or of thoughts, &c. in words)

express all other intensities . For example, electric lights are classified according to See also:

STANDARDS

Standards of Light. their " candle-power." The candle, in terms of whose brightness the brightness of other sources of light is to be expressed, must, of course, fulfil the conditions demanded of all standards . It must give under definite and easily realizable conditions a definite and See also:

CONSTANT, BENJAMIN (1845-1902)

constant luminous effect, and it must be easily reproducible . The earlier attempts to get a candle of constant brightness were not very satisfactory . The See also:

BRITISH

British See also:

standard is a sperm candle which weighs Ib, and loses in burning 120 grains per See also:

HOUR

hour . It is found that these conditions are not sufficient to determine the luminous power of the candle, since the length and shape of the See also:

WICK

wick, the height of the See also:

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

flame, and the See also:

composition, temperature and humidity of the See also:

ATMOSPHERE (Gr. fir µ6r, vapour; orkaipa, a sphere)

atmosphere all have an effect upon its brightness . The same is true of other similar sources of light—for example, the See also:

German standard candle, which is made of See also:

PARAFFIN

paraffin, has a See also:

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

diameter of 2 cm., and has its wick cut until the flame is 5 cm . high, but which with all precautions suffers continual altera- tions in brightness .

For See also:

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

ordinary practical purposes, however, these candles are steady enough . Other kinds of flame have also been used as a standard source of light . The See also:

OLDEST

oldest of these is the See also:

French Carcel See also:

LAMP (from Gr. Xag ras, a torch, Xaµaav, to shine)

lamp, which is provided with a cylindrical Argand burner, and gives the standard brightness . See also:

Vernon- when 42 grammes of colza oil are consumed per hour . See also:

Harcourt The See also:

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

supply and See also:

DRAUGHT (from the common Teutonic word " to draw "; cf. Ger. 7'racht, load; the pronunciation led to the variant form " draft," now confined to certain specific meanings)

draught are regulated by clockwork . Pentane A . G . Vernon-Harcourt's pentane standard, in which standard. a mixture of gaseous pentane and See also:

air is burnt so as to maintain a flame 2'5 in. high at ordinary barometric pressure, gives See also:

GOOD, JOHN MASON (1764-1827)

good results, and is readily adjustable to suit varied See also:

con- ditions . Several forms of this standard have been constructed, one of the most important being the ro candle-power pentane lamp, in which air saturated with pentane vapour is burnt in a specially-designed burner resembling an Argand burner . For photometric purposes a definite length of the See also:

LOWER

lower part of the flame is used, the upper part being hidden within an opaque See also:

TUBE (Lat. tuba)

tube . The amyl-acetate lamp designed by H. von Hefner-Alteneck has been elaborately studied by the German authorities, and at See also:

PRESENT

present is probably more used than any other flame for photometry . It is of See also:

SIMPLE

simple construction, and gives the standard brightness when it See also:

burns with a flame 4 cms. in height in still air of humidity o.88% and See also:

FREE

free of See also:

CARBON (symbol C, atomic weight 12)

carbon dioxide .

The presence of carbon dioxide and increase in the humidity have a marked effect in diminishing the brilliancy of the flame . If the vapour pressure is e and the barometric pressure p, the strength of the flame, when all other conditions are fulfilled, is given by the See also:

formula r'049 — 5' 5c/(p — e) One disadvantage for photometric purposes is the reddish colour of the flame as compared with the whiter artificial lights in general use . For an interesting See also:

account of the various experimental investigations into the properties of the Hefner flame see E . L . See also:

NICHOLS, JOHN (1745–1826)

Nichols, Standards of Light," Transactions of the See also:

INTERNATIONAL, THE

International See also:

Electrical See also:

CONGRESS (Lat. congressus, coming together, from congredi; cum, with, and gradus, step)

Congress, vol. ii . (St See also:

Louis, 1904).1904) . See also:

ANGSTROM, ANDERS JONAS (1814-1874)

Angstrom's determination of the radiation of the flame in See also:

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

absolute energy See also:

units is also of See also:

SPECIAL

special See also:

INTEREST

interest . Attempts have been made, but hitherto with limited success, to construct a convenient standard with See also:

ACETYLENE

acetylene flame . Could a satisfactory burner be devised, so that a steady brilliancy could be easily maintained, acetylene would, because of its intense white light, soon displace all other flames as standards . J . Violle has proposed to use as standard the light emitted by a square centimetre of surface of See also:

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

platinum at its melting point, but there are obvious practical difficulties in the way of realizing this suggested standard . J .

E . Mlle's Petavel, who carefully examined the necessary condi- Platinum tions for producing it (Prot . See also:

ROY, WILLIAM (1726-1790)

Roy . See also:

Soc . 1899), finds Standard. that the platinum must be chemically pure, that the crucible must be made of pure See also:

lime, that the See also:

FUSION

fusion must be by means of the oxy-See also:

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

hydrogen See also:

BLOW, JOHN (1648-1708)

blow-See also:

PIPE

pipe, that the gases must be thoroughly mixed in the proportion of 4 volumes of hydrogen to 3 of See also:

OXYGEN (symbol 0, atomic weight 16)

oxygen, and that the hydrogen must contain no hydro-carbons . Under these cohditions the variation in the light emitted by the molten platinum would probably not exceed 1% . O . Lummer and F . Kurlbaum have proposed as a standard a See also:

STRIP

strip of platinum See also:

FOIL

foil 25 mm. wide and •015 mm. thick brought to incandescence by an electric current of about 8o amperes . The temperature is gradually increased until 1lo-th of the total radiation is transmitted through a See also:

WATER

water trough 2 cm. in width . This ratio is determined by means of a bolometer, and so long as it is adjusted to - th the light is practically constant . For See also:

COMPARATIVE

comparative photometric See also:

WORK

work the incandescent. electric light is very convenient, having the one great See also:

ADVANTAGE

advantage over candles and flames that it is not affected by atmospheric changes .

But it does not satisfy the requirements of a See also:

PRIMARY

primary standard . It ages with use, and when run at constant voltage gradually loses in brilliancy, partly because of changes in the filament itself, partly because of the See also:

DEPOSIT (Lat. depositurn, from deponere, to lay down, to put in the care of)

deposit of carbon on the interior of the bulb . Professor J . A . See also:

Fleming has shown that very good results can be obtained if carbon filaments carefully selected Fleming's and run in ordinary bulbs for a definite See also:

time at a Incandeslittle above their normal voltage are remounted centLamp in large clear See also:

glass bulbs 6 or 8 in. in diameter . Standard . If used sparingly, and never above their marked voltage, these large incandescent bulbs have been found to remain constant for years, and therefore to be eminently suitable as secondary standards . In his Handbook for the Electrical Laboratory and Testing See also:

ROOM

Room (vol. ii.) Fleming concludes that the best primary standards are the Violle incandescent platinum and the Vernon-Harcourt pentane one-candle flame; and that the most convenient practical standards are the Hefner lamp, the ten-candle pentane lamp, and the Fleming large bulb incandescent electric lamp . Comparisons of the intensities of these various standards do not give quite concordant results . Thus three different authorities have estimated the 10-candle pentane lamp as being equal to 10.75, 11.0, 11.4 Hefner lamps . A specially constructed See also:

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

instrument or piece of apparatus for comparing light intensities or illuminations is called a Talbot's Law . Hefner Lamp .

photometer . The earlier forms of photometers were very simple and not capable of giving very precise results . The principles of Photo construction are, however, the same in all the recog- meters. nized forms down to the most elaborate of recent inventions . Two of the earliest forms were described by P . See also:

BOUGUER, PIERRE (1698-1758)

Bouguer and W . See also:

Ritchie . The Ritchie See also:

WEDGE (O. Eng. wecg, a mass of metal, cognate with Dutch wig, wigge, Dan. vaegge, &c.; in Lith. the cognate form outside Teut. is found in wagis, a peg, spigot; there is no connexion with " weigh," " weight," which must be referred to the root wegh, to li

wedge constitutes the basis of many varieties of type . The two lights to be Rltchte's compared illuminate the sides of the wedge, which wedge,. is placed between them, so that the eye set in front of the wedge See also:

SEES

sees the two sides illuminated each by one of the lights . The edge should be as See also:

sharp as possible so that the two illuminated surfaces are in See also:

close contact . The illuminations are made equal either by shifting the wedge along the line joining the lights or by moving one of the lights nearer to or farther from the wedge as may be required . The lights given out by the sources are then as the squares of the distances from See also:

RUMFORD, BENJAMIN THOMPSON, COUNT (1753-1814)

Rumford's the matched parts of the surfaces . See also:

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

Count Rumford Photo-* suggested the comparison of the intensity of the See also:

METER, ELECTRIC

meter. shadows of the same See also:

OBJECT

object thrown side by side on a screen by the two lights to be compared .

In this See also:

case the See also:

SHADOW (0. Eng. Schadewe, sceadu; a form of " shade "; connected with Gr. 6-Ore's, darkness)

shadow due to one source is lit up by the other alone; and here again the amounts of light given out by the sources are as the squares of their distances from the screen when the shadows are equally intense . The shadow-casting object should be near the screen, so as to avoid See also:

PENUMBRA (Lat. paene, almost, umbra, a shadow)

penumbra as much as possible; yet not too near, so that the shadows may not overlap . R . See also:

Bunsen suggested the very simple expedient of making a grease-spot on white See also:

paper for photometric purposes . When Bunsen's the paper is equally illuminated from both sides Photo- the grease-spot cannot be seen except by very meter. close inspection . In using this photometer, the sources are placed in one line with the grease-spot, which lies between them and can be moved towards one or other . To make the most accurate determinations with this arrangement the See also:

ADJUSTMENT (from late Lat. ad juxtare, derived from juxta, near, but early confounded with a supposed derivation from justus, right)

adjustment should first be made from the side on which one source lies, then the screen turned round and the adjustment made from the side of the other source—in both cases, therefore, from the same side of the paper screen . Take the mean of these positions (which are usually very close together), and the amounts of light are as the squares of the distances of the sources from this point . The efficiency of the Bunsen photometer has been improved by using two inclined mirrors so that the eye views both sides of the paper simultaneously . Sir See also:

Charles See also:

WHEATSTONE, SIR CHARLES (1802–1875)

Wheatstone suggested a hollow glass See also:

BEAD

bead, silvered internally, and made to describe very rapidly a closed path, for See also:

Wheat- use as a photometer . When it is placed between two See also:

stone'sPho- sources we see two parallel curves of reflected light, rometen one due to each source . Make these, by trial, equally bright; and the amounts of light from the sources are, again, as the squares of the distances .

William Swan's prism photometer, invented in 1859, is a beautiful application of the principle embodied in Bunsen's grease-spot photo-Swan's meter (see Trans . Roy . Soc . Ed. vol. xxi.) . The essential Swan part of the instrument is fundamentally the same as See also:

Double pm that described by O . Lummer and E . Brodhun in 1889 . It consists of two equal right-angled isosceles glass prisms placed with their See also:

DIAGONAL (Gr. &a, through, ywvia, a corner)

diagonal faces together so as to form a See also:

CUBE (Gr. K46os, a cube)

cube (fig . I), and cemented together by a small patch of See also:

CANADA

Canada See also:

BALSAM (from Gr. f &X raµov, through Lat. balsamum, contracted by popular use to O. Fr. basme, mod. Fr. bdme; Eng. balm)

balsam, which spreads out into a circle when the prisms are pressed together . In the figure, which represents a central See also:

section of the bi-prism, the Canada balsam is represented by the See also:

LETTER (through Fr. lettre from Lat. littera or litera, letter of the alphabet; the origin of the Latin word is obscure; it has probably no connexion with the root of linere, to smear, i.e. with wax, for an inscription with a stilus)

letter N . The , light from two illuminated surfaces, PQ, RS, is allowed to fall perpendicularly on the faces AB, 0 AD . In each case that part of the light falling internally on the portion of the diagonal See also:

FACE (from Lat. fades, derived either from facere, to make, or from a root fa-, meaning " appear "; cf. Gr. cbatvstv)

face which is not backed with the Canada balsam is totally reflected .

On the other See also:

hand, the light which falls on the portion backed by the 4 P Canada balsam is almost wholly transmitted . Thus an eye placedin the position qtp receives light from both sources, the surface RS supplying nearly all the light that seems to come from the patch N, and the surface PQ supplying all the light which seems to come from the region immediately surrounding N . The patch N will in general be visible; but it will quite disappear when the luminosity of the ray Tt, which traverses the Canada balsam, is exactly equal to the luminosity of the rays Pp, Qq, which have come after total reflection from the surface PQ . This condition of in-visibility of N is arrived at by adjusting the positions of the sources of light which illuminate the surfaces PQ, RS . The brightnesses of the two sources will then be as the squares of their distances from their respective screens . The essential part of Lummer and Brodhun's photometer is a See also:

COMBINATION (Lat. combinare, to combine)

combination of prisms very similar to Swan's . In its most improved form the bi-prism or " See also:

OPTICAL

optical cube " has one 4umnd of its component prisms cut in a See also:

PECULIAR

peculiar manner . Brodmhuern'as The diagonal face is partly cut away, so that the central photometer. part only of this face can be brought into contact with the diagonal face of the other prism . The Canada balsam is dispensed with, the surfaces being pressed closely together so that no layer of air is See also:

LEFT

left between them . In See also:

order to make the instrument convenient for use with an optical See also:

BENCH (an O.E. and Eng. form of a word common to Teutonic languages, cf. Ger. Bank, Dan. baenk and the Eng. doublet " bank ")

bench, Lummer and Brodhun make the illuminated surfaces which are to be compared the opposite sides of an opaque screen set in the continuation of the diagonal (CA) of the bi-prism, the rays being brought by reflection from symmetrically situated mirrors so as to enter the sides AB and AD perpendicularly . An important modification, due also to Lummer and Brodhun, is the following: By means of a See also:

sand-blast a portion, which may be called r, is removed from one See also:

HALF

half of the diagonal face of the one prism, and from the other half of the same prism there is removed in like manner all but a part 1 corresponding to the part r . The portions which have not been removed are pressed close to the diagonal face of the other prism, and become the parts through which light is freely transmitted .

On the other hand, the light which enters the second prism and falls on the portions of surface backed by the layers of air filling the cut-out parts is totally reflected . The general result is the See also:

PRODUCTION (Lat. productionem, from producere, to pro-duce)

production of two similar luminous patches 1 and r, each of which is surrounded by a See also:

field of the same intensity as the other patch . When the photometric match is made the whole region will be uniformly bright . But, by insertion of strips of glass so as to weaken equally the intensity in the surrounding See also:

FIELDS, JAMES THOMAS (1817-1881)

fields, the match will be obtained when these fields are made of equal intensity and when at the same time the two patches differ equally in intensity from them . Under these conditions the eye is able to judge more certainly as to the equality of intensity of the two patches, and an untrained observer is able to effect a comparison with an accuracy which is impossible with most forms of photometer . J . Joly's See also:

DIFFUSION (from the Lat. diffundere; dis-, asunder, and fundere, to pour out)

diffusion photometer consists of two equal rectangulat parallelepipeds of a translucent substance like paraffin separated by a thin opaque disk . It is set between the sources Joly,s of light to be compared in such a way that each paraffin photometer See also:

block is illuminated by one only of the sources, and is adjusted until the two blocks appear to be of the same brightness . The method is made more sensitive by mounting the photometer on an elastic vibrator so as to render it capable of a slight to-and-fro oscillation about a mean position . A form of photometer which is well adapted for measuring the illumination in a region is that due to L . See also:

Weber . It consists of a See also:

HORIZONTAL

horizontal tube across one end of which is fitted another Weber's tube at right angles .

This second tube can be rotated Photometer. into any position perpendicular to the horizontal tube . Where the axes of the two tubes meet is placed in the later forms of the instrument one of Lummer and Brodhun's modified Swan cubes . At the other end of the horizontal tube a standard flame is set illuminating a piece of ground glass which may be moved to any convenient position in the tube . The eye looks along the See also:

CROSS

cross tube, at the farther end of which is placed another piece of ground glass illuminated from the outside . The illuminations of the two pieces of ground glass as viewed through the photometer double prism are brought fo equality, either by shift of the ground glass to or from the standard light, or by means of two Nicol prisms placed in the cross tube . One advantage of the instrument is its portability . The photometry of incandescent electric lamps has led to several special modifications and devices . The candle power varies distinctly in different horizontal directions, !mans and one measurement in any particular direction descent is not sufficient . Sometimes the lamp is rotated Etect'ac about three times a second about a See also:

VERTICAL (from Lat. vertex, highest point)

vertical See also:

AXIS (Lat. for " axle ")

axis and Lamp . an See also:

AVERAGE

average value thus obtained . But there is always a See also:

RISK

risk of the filament breaking; and in all cases the effect of centrifugal force must alter the form of the filament and therefore the distances of the different parts from the screen . Accuracy demands either the measurement of the radiation intensity in a number of directions all round the lamp, or one combined R TS measurement of as many rays as can be conveniently combined .

One of the best methods of effecting this is by means of See also:

MATTHEWS, STANLEY (1824-1889)

Matthews's C . P . Matthews's integrating photometer . By the Integrating use of twelve mirrors arranged in a semicircle whose Photometer.diameter coincides with the axis of the lamp, twelve rays are caught and reflected outward to a second set of twelve mirrors which throw the rays on to the surface of a photometric screen . This combined effect is balanced by the illumination produced by a standard lamp on the other side of the screen (see Trans . Amer . Inst . Elect . Eng., 1902, vol. xix.) . So long as the lights to be compared are of the same or nearly the same tint, the photometric match obtained by different Netero- observers is practically the same . If, however, they See also:

CHROMATIC (Gr. xpcoµaruc6s, coloured, from xpwµa, colour)

chromatic are of distinctly different See also:

COLOURS, MILITARY

colours, not only do dif-Photometry.ferent observers obtain different results but those obtained by the same observer at different times are not always in agreement . Helmholtz was of See also:

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

opinion that photo-metric comparison of the intensities of different coloured lights possessed no real intrinsic value .

There can be little doubt that in a rigorous sense this is true . Nevertheless it is possible under certain conditions to effect a comparison which has some practical value . For example, when the intensities of two differently coloured lights differ considerably there is no difficulty in judging which is the stronger . By making the one light pass through a fairly large range of brightness we may easily assign limits outside which the intensities are undoubtedly different . After some experience these limits get close; and many experimenters find it possible, by taking proper precautions, not only to effect a match, but to effect practically the Abney's same match time after time . According to Abney, See also:

EXPERT (Lat. expertus, from experiri, to try)

expert• whose See also:

MEMOIRS

memoirs on colour photometry (Phil . Trans., ments . 1886, 1892) form a most important contribution to the subject, the observer in making his See also:

JUDGMENT

judgment as to the equality of luminosity of two patches of colour placed side by side must not begin to think about it, but must let the eye See also:

ACT (Lat. actus, actum)

act as unconsciously as possible . His method was to compare the coloured patch with white light given by a particular standard and cut down to the proper intensity by use of a Talbot's rotating sector, which could be adjusted by means of a suitable mechanism while it was rotating . At the same time, although the eye may be able to effect a definite matching of two patches of colour of a particular luminosity, it has been long known that a 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 in the luminosity will destroy the apparent equality . This depends upon a physiological property of the retina discovered by J . E .

Purkinje in 1825 (see below, See also:

CELESTIAL, OR STELLAR

Celestial Photometry) . In virtue of this property the See also:

BLUE (common in different forms to most European languages)

blue and See also:

VIOLET

violet end of the spectrum is more stimulating to the eye than the red end when the general luminosity is See also:

low, whereas at high luminosities the red gains relatively in brightness until it becomes more stimulating than the blue . Unless therefore account is taken in some definite measurable manner of the absolute brightness, there must always be some uncertainty in the photometric comparison of the intensities of differently coloured sources of light . Instead, however, of trying to effect a photometric match in any of the ways which have been found sufficient when the sources are of the same or nearly the same tint, we may effect important practical comparisons in what is called heterochromatic photometry by an See also:

APPEAL

appeal to other physiological properties of the eye . For example, the power of clearly discriminating patterns in differently coloured lights of various intensities is obviously of great practical importance; and this power of detailed discrimination may be made the basis of a method of photometry . According to this method two lights Discrimina- are arranged so as to illuminate two exactly similar 'See also:

Ion Photo-patterns of lines See also:

DRAWN

drawn, for example, on the sides meter. of a Ritchie wedge, and their distances are adjusted until the patterns are seen equally distinct on the two sides . Application of the usual distance law will then give the relation between the two lights . A discrimination photometer constructed on this principle has been designed by J . A . Fleming . Its results do not agree with the indications of an ordinaryluminosity photometer; for it is found that the eye can discriminate detail better with yellow than with blue light of the same apparent luminous intensity . Another and very promising method of photometry depends upon the duration of luminous impressions on the retina .

J . A . F . See also:

Plateau observed in 1829 that the blending into Flicker a homogeneous impression of a See also:

PATTERN

pattern of alternatePhotometry, sectors of See also:

black and some other colour marked on a disk when that disk was rotated occurred for rates of rotation which depended on the colour used . A form of experiment suggested in Professor O . N . See also:

ROOD (O.E. rod, a stick, another form of " rod, O.E. rodd, possibly cognate with Lat. rudis, a staff)

Rood's See also:

MODERN

Modern Chromatics seems to have been first carried out by E . L . Nichols (Amer . Journ. of Science, 1881) . A black disk with four narrow open sectors was rotated in front of the slit of a spectroscope . When the rotation was not too See also:

QUICK

quick the yellow part of the spectrum appeared as a See also:

SUCCESSION (Lat. successio, from succedere, to follow after)

succession of flashes of light separated by intervals of darkness of appreciable length, whereas towards both the red and violet ends no apparent interruption in the steady luminosity could be observed .

As the See also:

RATE

rate of rotation increased the part of the spectrum in which flickering appeared contracted to a smaller length extending on each side of the yellow, and finally with sufficiently rapid See also:

ALTERNATION (from Lat. alternare, to do by turns)

alternation the yellow itself became steady . This seems to show that the retinal See also:

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

image persists for a shorter time with yellow light than with light of any other colour; for with it the intervals of darkness must be shorter before a continuous impression can be obtained . Now yellow is the most luminous part of the spectrum as it affects the normal human eye; and E . S . See also:

Ferry (Amer . Journ. of Science,1892) has shown that the duration of luminous impression is mostly, if not entirely, determined by the luminosity of the ray . Hence the determination of the minimum rate of intermittence at which a particular colour of light becomes continuous may be regarded as a measure of the luminosity, the slower rate corresponding to the lower luminosity . Although in the experiment just described the red part of the ordinary See also:

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

solar spectrum becomes continuous for a slower rate of intermittence than the yellow part, yet we have simply to make a red ray as luminous as the yellow ray to find that they become continuous for the same rate of intermittence . It is, however, highly improbable that the duration of impression depends only on the luminosity of the light and not to some extent upon the See also:

WAVE

wave-length . There are indeed phenomena which require for their explanation the See also:

ASSUMPTION, FEAST OF

assumption that the duration of luminous impression does depend on the colour as well as on the brightness . Nevertheless the luminosity is by far the more important See also:

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

factor, as shown by See also:

OGDEN

Ogden N . Rood's experiments .

He found (Amer . Journ. of Science, 1893) that, when a disk whose halves , Ex. differ in tint but not in luminosity is rotated rather Roods periments . s slowly, the eye of the observer sees no flickering such as is at once apparent when the halves differ slightly in luminosity . Rood himself suggested various forms of photo-meter based on this principle . In his latest form (Amer . Journ. of Science, See also:

SEPT

Sept . 1899) the differently coloured beams of light which are to be compared photometrically are made to illuminate the two surfaces of a Ritchie wedge set facing the eye . Between the wedge and the eye is placed a cylindrical See also:

CONCAVE

concave See also:

lens, which can be set in oscillation by means of a motor in such a way that first the one illuminated surface of the wedge and then the other is presented to the eye in sufficiently rapid alternation . The one source of light is kept fixed, while the other is moved about until the sensation of flicker disappears . From work with this form of instrument Rood concluded that " the accuracy attainable with the flicker photometer, as at present constructed, and using light of different colours almost spectral in See also:

hue, is about the same as with ordinary photometers using See also:

PLAIN (O. Fr. plain, from Lat. plenum)

plain white light, or light of exactly the same colour . Various modifications of Rood's forms have been constructed from time to time by different experimenters . Thesimmanee Simmance and Abady flicker photometer is an ingeniousandAbady's and yet mechanically simple method by which (as it photometer. were) the wedge itself is made to oscillate so as to throw on the eye in rapid succession, first the one side and then the other .

The rim of a See also:

wheel of white material is bevelled in a peculiar manner . The sharp edge, which passes slightly obliquely across the rim from One side of the wheel to the other and back again, is the 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 two exactly similar conical surfaces facing different ways and having their axes parallel to, but on opposite sides of, the axis of rotation of the wheel . As the wheel rotates with its rim facing the eye, the intersection of the two surfaces crosses and recrosses the line of See also:

VISION (from Lat. videre, to see), or SIGHT

vision during each revolution . Hence first the one illuminated side and then the other are presented to the eye in rapid alternation . The inventors of this instrument claim that their instrument can See also:

GAUGE, or GAGE (Med. Lat. gauja, jaugia, Fr. jauge, perhaps connected with Fr. jale, a bowl, galon, gallon)

gauge accurately and easily the relative intensities of two lights, whether of the same or of different colour (Phil . Mag., 1904) . There is no doubt that results obtained by different observers with a flicker photometer are in better agreement than with any other form of photometer . The comparative ease with which the See also:

BALANCE (derived through the Fr. from the Late Lat. bilantia, an apparatus for weighing, from bi, two, and lanx, a dish or scale)

balance is obtained even when the tints are markedly different shows that its See also:

ACTION

action depends upon a visual distinction which the eye can readily appreciate, and this distinction is mainly one of brightness . The spectrophotometer is an instrument which enables us to make photometric comparisons between the similarly coloured spectra. portions of the spectra of two different sources of photometry. light, or of two parts of the same See also:

ORIGINAL

original source after they have passed through different absorbing See also:

MEDIA

media . When it is desired to compare the intensities of the spectra from two different sources a convenient form is the one described by E . L . Nichols .

A direct visior_ spectroscope mounted upon a See also:

CARRIAGE

carriage travels along a track between the two sources . In front of the slit two right-angled triangular prisms are set so that the light from each source enters the one side of one prism perpendicularly and is totally reflected into the spectroscope . The two spectra are then seen side by side . See also:

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

Attention being fixed on some chosen narrow portion, say, in the See also:

green, the instrument is moved along the track between the sources until the two portions appear of the same intensity . The See also:

PROCESS

process is then repeated until the whole spectrum has been explored . In Lummer and Brodhun's form of spectrophotometer the rays to be compared pass in perpendicular lines through the modified See also:

Brace's Swan double prism, and then together side by side Bpctro- through a spectroscope . By means of a simple modifi- photometer. cation in the form of the two prisms, Professor D . B . Brace (Phil . Mag., 1899) made the combined prism serve to produce the spectra as well as to effect the desired comparison . In this arrangement the See also:

compound prism See also:

ABC (fig . 2) is made up of two equal right-angled prisms S See also:

ADB and ADC placed with their longer sides in contact, so that the whole forms an equilateral prism with three polished faces .

Part of the interface AD is silvered, the silvering forming a narrow central strip See also:

RUNNING

running parallel to AD . Along the 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 of the interface the two prisms are cemented together with Canada balsam or other material having as nearly as See also:

INDEX

index as the glass . When two rays R S enter symmetrically from opposite sides of the See also:

BASE

base of the compound prism as shown in the See also:

DIAGRAM

diagram, the ray R will pass through the prism except where the See also:

SILVER

silver strip intercepts it, and will form a part of a spectrum visible to the eye placed at R', while to the same eye there will be visible the similarly dispersed ray SS' reflected from the silvered surface . Thus two systems of incident parallel rays of white light will form on emergence two spectra with corresponding rays exactly parallel . With these and other forms of instrument the aim of the experimenter is to make the two spectra of equal intensity by a method which enables him to compare the original intensities of the sources . In most cases the relative intensities of the portions of the spectra being compared cannot conveniently be altered by varying the distances of the sources . Recourse is therefore generally had to one of the other methods already mentioned, such as the use of polarizing prisms or of rotating sectors . Under certain conditions K . Vierordt's method of allowing the two rays to pass through slits of different width leads to good results, but too great confidence cannot be placed upon it . In other types of spectrophotometer, such as those associated with the names of H . Trannin, A . Crova, H .