INSTRUMENTS  , &C . We proceed to give an See also:

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

account of the methods and principles of construction of the various kinds of telescopes, and See also:

• AYSCOUGH, SAMUEL (1745-1804)

Ayscough was an optician in Ludgate See also:

• HILL
• HILL (0. Eng. hyll; cf. Low Ger. hull, Mid. Dutch hul, allied to Lat. celsus, high, collis, hill, &c.)
• HILL, A
• HILL, AARON (1685-175o)
• HILL, AMBROSE POWELL
• HILL, DANIEL HARVEY (1821-1889)
• HILL, DAVID BENNETT (1843–1910)
• HILL, GEORGE BIRKBECK NORMAN (1835-1903)
• HILL, JAMES J
• HILL, JOHN (c. 1716-1775)
• HILL, MATTHEW DAVENPORT (1792-1872)
• HILL, OCTAVIA (1838– )
• HILL, ROWLAND (1744–1833)
• HILL, SIR ROWLAND (1795-1879)

Hill, See also:

• LONDON

London . to describe in detail See also:

• SPECIAL

special typical instruments, which, owing to the See also:

• WORK

work accomplished by their aid or the See also:

• PRACTICAL

practical advances exemplified in their construction, appear most worthy of See also:

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

record or study . Refracting See also:

• TELESCOPE

Telescope In its simplest See also:

• FORM (Lat. forma)

form the telescope consists of a See also:

• CONVEX

convex See also:

• OBJECTIVE, or OBJECT GLASS

objective capable of forming an See also:

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

image of a distant See also:

• OBJECT

object and of an See also:

• EYE
• EYE (O. Eng. edge, Ger. Auge; derived from an Indo-European root also seen in Lat. oc-ulus, the organ of vision (q.v.)

eye-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, See also:

• CONCAVE

concave or convex, by which the image so formed is magnified . When the See also:

• AXIS (Lat. for " axle ")

axis of the eye-lens coincides with that of the object-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, and the See also:

• FOCAL

focal point of the eye-lens is coincident with the See also:

• PRINCIPAL

principal See also:

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

focus of the object-lens, parallel rays incident upon the object-glass will emerge from the eye-piece as parallel rays . These, falling in turn on the lens of the human eye, are converged by it and form an image on the retina . Fig . I shows the course of the rays when the eye-lens is convex (or See also:

• POSITIVE (or PORTABLE) ORGAN

positive), fig . 2 when the eye-lens is concave (or negative) . The former represents See also:

• KEPLER, JOHANN (1571-1630)

Kepler's, the latter Lippershey's or the Galilean telescope . The magnifying See also:

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

power obviously depends on the proportion of the focal length of the object-lens to that of the eye-lens, that is, magnifying power=F/e, where F is the focal length of the object-lens and e that of the eye-lens . Also the See also:

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

diameter of the See also:

• PENCIL (Lat. penicillus, brush, literally little tail)

pencil or parallel rays emerging from the eye-lens is to the diameter of the object-lens inversely as the magnifying power of the telescope .

Hence one of the best methods of determining the magnifying power of a telescope is to measure the diameter of the emergent pencil of rays, after the telescope has been adjusted to focus upon a See also:

• STAR

star, and to See also:

• DIVIDE

divide the diameter of the object-glass by the diameter of the emergent pencil . If we See also:

• DESIRE

desire to utilize all the parallel rays which fall upon an object-glass it is necessary that the full pencil of emerging rays should enter the observer's eye . Assuming with See also:

• SIR

Sir See also:

• WILLIAM
• WILLIAM (1143-1214)
• WILLIAM (1227-1256)
• WILLIAM (1J33-1584)
• WILLIAM (A.S. Wilhelm, O. Norse Vilhidlmr; O. H. Ger. Willahelm, Willahalm, M. H. Ger. Willehelm, Willehalm, Mod.Ger. Wilhelm; Du. Willem; O. Fr. Villalme, Mod. Fr. Guillaume; from " will," Goth. vilja, and " helm," Goth. hilms, Old Norse hidlmr, meaning
• WILLIAM (c. 1130-C. 1190)
• WILLIAM, 13TH

William See also:

• HERSCHEL, CAROLINE LUCRETIA (1750-1848)
• HERSCHEL, SIR F
• HERSCHEL, SIR FREDERICK WILLIAM (1738-1822)
• HERSCHEL, SIR JOHN FREDERICK WILLIAM, BART

Herschel that the normal See also:

• PUPIL (Lat. pupillus, orphan, minor, dim. of pupus, boy, allied to puer, from root pm- or peu-, to beget, cf. "pupa," Lat. for " doll," the name given to the stage intervening between the larval and imaginal stages in certain insects)

pupil of the eye distends to one-fifth of an See also:

• INCH (O. Eng. ynce from Lat. uncia, a twelfthpart; cf. " ounce," and see As)

inch in diameter when viewing faint See also:

• OBJECTS

objects, we obtain the See also:

• RULE

rule that the minimum magnifying power which can be efficiently employed is five times the diameter of the object-glass expressed in inches.' The defects of the Galilean and Kepler telescopes are due to the See also:

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

chromatic and spherical See also:

• ABERRATION (Lat. ab, from or away, errare,, to wander)

aberration of the See also:

• SIMPLE

simple lenses of which they are composed . The substitution of a positive or negative eye-piece for the simple convex or concave eye-lens, and of an achromatic object-glass for the simple object-lens, transforms these See also:

• EARLY
• EARLY, JUBAL ANDERSON (1816-1894)

early forms into the See also:

• MODERN

modern achromatic telescope . The Galilean telescope with a concave eye-lens instead of an eye-piece still survives as the modern See also:

• OPERA (Italian for " work ")

opera-glass, on account of its shorter length, but the object-glass and eye-lens are achromatic combinations . (D . GI.) Telescope Objectives.3-In spite of the improvements in the manufacture of See also:

• OPTICAL

optical glass (see GLASS) practically the same 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 glasses as used by See also:

• JOHN
• JOHN (1167–1216)
• JOHN (1290-c. 1320)
• JOHN (1296-1346)
• JOHN (1371–1419)
• JOHN (1468-1532)
• JOHN (1801-1873)
• JOHN (Heb. llni')
• JOHN (ZAPOLYA) (1487-1540)
• JOHN, 4TH MARQUESS OF TWEEDDALE (c. 1695-1762)
• JOHN, DON (1545-1578)
• JOHN, DON (1629–1679)
• JOHN, GOSPEL OF ST
• JOHN, or HAYS (1513–1571)
• JOHN, THE APOSTLE
• JOHN, THE EPISTLES OF

John See also:

• DOLLOND, JOHN (1706—1761)

Dollond in 1758 for achromatic objectives are still used for all the largest of the modern refracting telescopes . It has See also:

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

long been known that the spectra of 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 or See also:

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

solar See also:

• LIGHT

light yielded by See also:

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

ordinary crown and flint glasses are different: that while two prisms of such glasses may be arranged to give exactly the same angular See also:

• DISPERSION
• DISPERSION (from Lat. dispergere, to scatter)

dispersion between two See also:

• FRAUNHOFER, JOSEPH VON (1787-1826)

Fraunhofer 2 In the See also:

• CASE
• CASE, JOHN (d. 1600)

case of See also:

• SHORT, FRANCIS JOB (1857– )

short-sighted persons the image for very distant objects (that is, for parallel rays) is formed in front of the retina; therefore, to enable such persons to see distinctly, the rays emerging from the eye-piece must be slightly divergent; that is, they must enter the eye as if they proceeded from a comparatively near object . For normal eyes the natural See also:

• ADAPTATION (from Lat. adaptare. to fit to)

adaptation is not to focus for quite parallel rays, but on objects at a moderate distance, and practically, therefore, most persons do adjust the focus of a telescope, for most distinct and easy See also:

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

vision, so that the rays emerge from the eye-piece very slightly divergent . Abnormally short-sighted per-sons require to push in the eye-lens nearer to the object-glass, and long-sighted persons to withdraw it from the See also:

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

adjustment employed by those of normal sight . It is usual, however, in computations of the magnifying power of telescopes, for the rays emerging from the eye-piece when adjusted for distinct vision to be parallel . 2 For the methods of grinding, polishing and testing lenses, see OBJECTIVE .

lines, such as C and F, yet the flint glass See also:

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

prism will show a relative See also:

• DRAWING

drawing out of the See also:

• BLUE (common in different forms to most European languages)

blue end and a crowding together of the red end of the spectrum, while the crown prism shows an opposite tendency . This want of proportion in the dispersion for different regions of the spectrum is called the " irrationality of dispersion "; and it is as a See also:

• DIRECT

direct consequence of this irrationality, that there exists a secondary spectrum or residual See also:

• COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

colour dispersion, showing itself at the focus of all such telescopes, and roughly in proportion to their See also:

• SIZE

size . These glasses, however, still hold the 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, although glasses are now produced whose irrationality of dispersion has been reduced to a very slight amount . The See also:

• PRIMARY

primary See also:

• REASON (Lat. ratio, through French raison)

reason for this retention is that nothing approaching the difference in dispersive power between ordinary crown glass and ordinary dense flint glass (a difference of 1 to II) has yet been obtained between any pair of the newer glasses . Consequently, for a certain focal length, much deeper curves must be resorted to if the new glasses are to be employed; this means not only greater difficulties in workmanship, but also greater thickness of glass, which militates against the See also:

• CHANCE (through the O. Fr. cheance, from the Late Lat. cadentia, things happening, from cadere, to fall out, happen; cf. " case ")

chance of obtaining large disks quite See also:

• FREE

free from striae and perfect in their See also:

• STATE
• STATE, GREAT OFFICERS OF

state of See also:

• ANNEALING, HARDENING AND

annealing . In fact, superfine disks of over 15 in. See also:

• APERTURE (from Lat. aperire, to open)

aperture are scarcely possible in most of the newer telescope glasses . Moreover the greater depths of the curves (or " curvature See also:

• POWERS, HIRAM (1805-1873)

powers ") in itself neutralize more or less the advantages obtained from the reduced irrationality of dispersion . When all is taken into 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 it is scarcely possible to reduce the secondary colour aberration at the focus of such a 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 object-glass to less than a See also:

• FOURTH

fourth See also:

• PART

part of that prevailing at the focus of a double objective of the same aperture and focus, but made of the ordinary crown and flint glasses . The only way in which the secondary spectrum can be reduced still further is by the employment of three lenses of three different sorts of glass, by which arrangement the secondary spectrum has been reduced in the case of the See also:

• COOKE, GEORGE FREDERICK (1756–1811)
• COOKE, JAY (1821–1905)
• COOKE, ROSE TERRY (1827-1892)

Cooke photo visual objective to about 1/loth part of the usual amount, if the whole region of the visible spectrum is taken into account . It is possible to construct a triple objective of two positive lenses enclosing between them one negative lens, the two former being made of the same glass . For relatively short focal lengths a triple construction such as this is almost necessary 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 obtain an objective free from aberration of the 3rd order, and it might be thought at first that, given the closest attainable degree of rationality between the colour dispersions of the two glasses employed, which we will See also:

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

call crown and flint, it would be impossible to devise another form of triple objective, by retaining the same flint glass, but adopting two sorts of crown instead of only one, which would have its secondary spectrum very much further reduced . Yet such is the rather surprising fact .

But it can be well illustrated in the case of the older glasses, as the following case will show . The figures given are the partial dispersions for ordinary crown and ordinary extra dense flint glasses, styled in Messrs Schott's See also:

• CATALOGUE (a Fr. adaptation of the Gr. KaraXoyos, a register, from Karalyety, to enrol or pick out)

catalogue of optical glasses as o•6o and 0.102 respectively, having refractive indices of 1.5179 and 1.6489 for the D See also:

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

ray respectively, and (µDr)/(µF--µc)=6o•2 and 33'8 respectively to indicate their dispersive powers (inverted), =v . C to F A to D D to F F to G * * * * o•6o •oo86o I•oo0 .00533 '643 '00605 •703 .00487 •566 0.102 .01919 1.000 .01152 •600 .01372 •714 •01180 .615 I .02779 ~ •613 I .7111•o1667 •600 1.000.01685 •01977 The Aµ from C to F being taken as unity in each case, then the c1 µ's for the other regions of the spectrum are expressed in fractions (C to F) and are given under the asterisks . Let it be supposed that two positive lenses of equal curvature powers are made out of these two glasses, then in order to represent the combined dispersion of the two together the two 0µ's for each spectral region may be added together to form A'µ as in the See also:

• LINE

line below, and then, on again expressing the partial t1'µin terms of D'µ (C to F) we get the new figures in the bottom See also:

• ROW, JOHN (c. 1525—1580)

row beneath the asterisks . We find that we have now got a course of dispersion or degree of rationality which very closely corresponds to that of an ordinary light flint glass, styled 0.569 in Schott's catalogue, and having µD 1.5738 and (µD-1)/(µF-µc) =41'4=v, the figures of whose course of dispersion are as below:- Light Flint Glass 0.569 . Hence it is clear that if the two positive lenses of equal curvature power of o•6o and 0•See also:

• IO2

IO2 respectively are combined with a negative lens of light flint o•569, then a triple objective, having no secondary spectrum (at any See also:

• RATE

rate with respect to the blue rays), may be obtained . But while an achromatic See also:

• COMBINATION (Lat. combinare, to combine)

combination of o•6o and o•102 alone will yield an objective whose focal length is only 1.28 times the focal length of the negative or extra dense flint lens, the triple combination will be found to yield an objective whose focal length is 73 times as See also:

• GREAT

great as the focal length of the negative light flint lens . Hence impossibly deep curvatures would be required for such a triple objective of any normal focal length . This case well illustrates the much closer approach to strict rationality of dispersion which is obtainable by using two different sorts of glass for the two positive lenses, even when one of them has a higher dispersive power than the glass used for the negative lens . It is largely to this principle that the Cooke photo visual objective of three lenses (fig . 3) owes its high degree of See also:

• ACHROMATISM (Gr. a-, privative, xpiaµa, colour)

achromatism . This form of objective has been successfully made up to 122 in. clear aperture .

The front lens is made of baryta light flint glass (o.543 of Schott's catalogue) and the back lens of a crown glass, styled 0.374 in Schott's older lists . The table gives their partial dispersions for six different regions of the spectrum also expressed (in brackets below) as fractional parts of the dispersion from C to F . C to F IAtoC DtoF'EtoFFtoG' FtoH 0.543 .01115 .00374 .Q0790 •00369 •00650 •01322 PD = 1 .564 (1.0000) (.3354) (.7085) (.3309) (.5830) (1.1857) v = 50 . 7 0'374 •00844 •00296 .00593 .00274 .00479 .00976 µD = 1.511 ( 1 .0000);(.3507) (.7026) (.3247) (.5675) (1.1564) v T6o•8 Since the curvature powers of the positive lenses are equal, the partial dispersions of the two glasses may be simply added together, and we then have:- [0'543+0'3741 CtoF AtoC DtoF EtoF FtoG' FtoH .01959 •00670 .01383 .00643 .01129 .02298 (1.0000) (.3420) (.7059) (.3282) (.5763) (1.1730) The proportions given on the See also:

• LOWER

lower line may now be compared with the corresponding proportional dispersions for borosilicate flint glass 0.658, closely resembling the type 0.164 of Schott's See also:

• LIST (O.E. lisle, a Teutonic word, cf. Dut. lust, Ger. Leiste, adapted in Ital. lista and Fr. lisle)
• LIST, FRIEDRICH (1789-1846)

list, viz.:- [o.658 (ND =1.546) v=50.11 CtoF Ato C 1.0000 •3425 CtoF A' to D D to F F to G .01385 I•000 .00583 .615 •00987 .713 .00831 •600 D to F E to F F to G' F to H •7052 .3278 .5767 1'1745 A slight increase in the relative power of the first lens of 0.543 would bring about a still closer See also:

• CORRESPONDENCE
• CORRESPONDENCE (from med. scholastic Lat. correspondentia, corrcspondere, compounded of Lat. cum, with, and respond ere, to answer; cf. Fr. correspondance)

correspondence in the rationality, but with the curves required to produce an object-glass of this type of 6 in. aperture and Io8 in. focal length a discrepancy of 1 unit in the 3rd decimal See also:

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

place in the above proportional figures would cause a linear See also:

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

error in the focus for that colour of only about .025 in., so that the largest deviation implied by the tables would be a focus for the extreme See also:

• VIOLET

violet H ray about •037 longer than the normal . It will be seen, then, that the visual and photographic foci are now merged in one, and the image is practically as achromatic as that yielded by a reflector . Other types of triple object-glasses with reduced secondary spectra have recently been introduced . The See also:

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

extension of the image away from the axis or size of field available for covering a photographic See also:

• PLATE

plate with See also:

• FAIR

fair See also:

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

definition is a See also:

• FUNCTION

function in the first place of the ratio between focal length and aperture, the longer focus having the greater relative or angular covering power, and in the second a function of the curvatures of the lenses, in the sense that the objective must be free from See also:

• COMA (Gr. K(.7)

coma at the foci of oblique pencils or must fulfil the sine See also:

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

condition (see ABERRATION) . Eye-pieces.—The eye-pieces or oculars through which, in case of visual observations, the primary images formed by the objective are viewed, are of quite secondary importance as regards definition in the central portion of the field of view . If an eye-piece blurs the definition in any degree in the centre of the field it must be very badly figured indeed, but the definition towards the edge of the field, say at 20° away from the centre of the apparent field of view, depends very intimately upon the construction of the eye-piece . It must be so designed as to give as See also:

• FLAT (a modification of O. Eng. flet, an obsolete word of Teutonic origin, meaning the ground beneath the feet)

flat an image as is possible consistently with freedom from astigmatism of oblique pencils . The See also:

• MERE
• MERE, ADALBERT (1838-1909)

mere size of the apparent field of view depends upon obtaining the oblique pencils of light emerging from it to See also:

• CROSS

cross the axis at the great possible 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, and to this end the presence of a field-lens is indispensable, which is separated from the eye-lens by a considerable See also:

• INTERVAL

interval . The earlier arrangement of two lenses of the Huygenian eye-piece (see See also:

• MICROSCOPE (Gr. µucp6s, small, asenreiv, to %view)

MICROSCOPE) having foci with ratio of 3 to I, gives a fairly large flat field of view approximately free from distortion of tangential lines and from coma, while the Mittenzwey variety of it (fig .

4) in which the field-lens is changed into a meniscus having radii in about the ratio of +I to – 9 gives still better results, but still not quite so See also:

• GOOD, JOHN MASON (1764-1827)

good as the results obtained by using the combination of two convexo-See also:

• PLANE

plane lenses of the focal ratio 2 to I . In the See also:

• RAMSDEN, JESSE (1735-1800)

Ramsden eye-piece (see MICROSCOPE) the focal lengths of the two piano-convex lenses are equal, and their convexities are turned towards one another . The field-lens is thus in the principal focal plane of the eye-lens, if the separation be equal to 2(Ji+.fi) . This is such a practical See also:

• DRAWBACK

drawback that the separation is generally ;ths or gths of the theoretical, and then the primary image viewed by the eye- piece may be rather outside the field-lens, which is a great practical See also:

• ADVANTAGE

advantage, especially when a reticule has to be mounted in the primary focal plane, although the edge of the field is not quite achromatic under these conditions . Kellner Eye-piece.—In order to secure the advantage of the principal focal plane of the eye-piece being well outside of the field-lens and at the same 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 to obtain a large flat field of563 view with oblique achromatism and freedom from coma and distortion, there is no better construction than the modified Kellner eye-piece (fig . 5) such as is generally used for prismatic binoculars. ft consists of a piano-convex field-lens of crown. glass and an approximately achromatic eye-lens, some distance behind it, consisting of an equi-convex crown lens cemented to a concavoplane flint lens, the latter being next to the eye . There are also other eye-pieces having the field-lens double or achromatic as well as the eye-lens . In cases where it is important to get the maximum quantity of light into the eye, the field-lens is discarded and an achromatic eye-lens alone employed . This yields a very much smaller field of view, but it is very valuable for viewing feeble telescopic objects and very delicate planetary or lunar details . Zeiss and Steinheil's monocentric eye-pieces and the Cooke single achromatic eye-piece (fig . 6) are examples of this class of oculars . (H .

D . T.) Reflecting Telescope . The following are the various forms of reflecting telescopes: The Gregorian telescope is represented in fig . 7, A A and B B are concave mirrors having a See also:

• COMMON

common axis and their concavities facing each other . The focus of A for parallel rays is at F, that of B for parallel rays at f—between B and F . Parallel rays falling on A A converge at F, where an image r. is formed; the rays are then reflected from B and converge at P, where a second and more enlarged image is. formed . See also:

• GREGORY
• GREGORY (Gregorius)
• GREGORY (Grigorii) GRIGORIEVICH ORLOV, COUNT (1734-1783)
• GREGORY, EDWARD JOHN (1850-19o9)
• GREGORY, OLINTHUS GILBERT (1774—1841)
• GREGORY, ST (c. 213-C. 270)
• GREGORY, ST, OF NAZIANZUS (329–389)
• GREGORY, ST, OF NYSSA (c.331—c. 396)
• GREGORY, ST, OF TOURS (538-594)

Gregory himself showed that, if the large See also:

• MIRROR (through O. Fr. mirour, mod. miroir, from a sup-posed Late Lat. miratorium, from mirari, to admire)

mirror were a segment of a paraboloid of revolution whose focus is F, and the small mirror an See also:

• ELLIPSOID

ellipsoid of revolution whose foci are F and P respectively, the resulting image will be plane and undistorted . The image formed at P is viewed through the eye-piece at E, which may be of the Huygenian or Ramsden type . The focal adjustment is accomplished by the See also:

• SCREW (O.E. scrue, from O. Fr. escroue, mod. ecrou; ultimate origin uncertain; the word, or a similar one, appears in Teutonic languages, cf. Ger. Schraube, Dan. skrue, but Skeat, following Diaz, finds the origin in Lat. scrobs, a ditch, hole, particularl

screw S, which acts on a slide carrying an See also:

• ARM (a common Teutonic word; the Indo-European root is ar, to join or fit; cf. the Lat. armus, shoulder, and the plural word arma, weapons, Gr. apphs, joint, and the reduplicated apapilKSV, to join)

arm to which the mirror B is attached . The practical difficulty of constructing Gregorian telescopes of good defining quality is very considerable, because if spherical mirrors are employed their aberrations tend to increase each other, and it is extremely difficult to give a true elliptic figure to the necessarily deep concavity of the small See also:

• SPECULUM

speculum . Short appears to have systematically conquered this difficulty, and his Gregorian telescopes attained great celebrity . The use. of the Gregorian form is, however, practically abandoned in the See also:

• PRESENT

present See also:

• DAY (O. Eng. dreg, Ger. Tag; according to the New English Dictionary, " in no way related to the Lat. dies")
• DAY, JOHN (1574-1640?)
• DAY, THOMAS (1748-1789)

day .

The magnifying power of the telescope is =Ff/ex, where F and f are respectively the focal lengths of the large and the small mirror, e the focal length of the eye-piece, and x the distance between the principal foci of the two mirrors (=Ff in the See also:

• DIAGRAM

diagram) when the See also:

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

instrument is in adjustment for viewing distant objects . The images are erect . The Cassegrain telescope differs from the Gregorian only in the substitution of a convex hyperboloidal mirror for a concave ellip- soidal mirror as the small speculum . This form has two Casae- distinct advantages: (i) if spherical mirrors are employed See also:

• CASS, LEWIS (1782–1866)

Cass their aberrations have a tendency to correct each other; grafts . (2) the instrument is shorter than the Gregorian, caeteris paribus, by twice the focal length of the small mirror . Fewer telescopes have been made of this than perhaps of any other form of reflector; but in comparatively See also:

• RECENT

recent years the Cassegrain has acquired importance from the fact of its See also:

• ADOPTION (Lat. adoptio, for adoptalio, from adoptare, to choose for oneself)

adoption for the great See also:

• MELBOURNE
• MELBOURNE, WILLIAM LAMB, 2ND VISCOUNT (1779-1848)

Melbourne telescope, and from its employment in the 6o-in. reflector of the See also:

• MOUNT, WILLIAM SIDNEY (1807-1868)

Mount See also:

• WILSON, ALEXANDER (1766-1813)
• WILSON, HENRY (1812–1875)
• WILSON, HORACE HAYMAN (1786–1860)
• WILSON, JAMES (1742—1798)
• WILSON, JAMES (1835— )
• WILSON, JAMES HARRISON (1837– )
• WILSON, JOHN (1627-1696)
• WILSON, JOHN (178 1854)
• WILSON, ROBERT (d. 1600)
• WILSON, SIR DANIEL (1816–1892)
• WILSON, SIR ROBERT THOMAS (1777—1849)
• WILSON, SIR WILLIAM JAMES ERASMUS
• WILSON, THOMAS (1663-1755)
• WILSON, THOMAS (c. 1525-1581)
• WILSON, WOODROW (1856— )

Wilson Solar See also:

• OBSERVATORY

Observatory (see below) . For spectroscopic purposes the Cassegrain form has See also:

• PECULIAR

peculiar advantages, because in consequence of the less rapid convergence of the rays after reflection from the convex hyperboloidal mirror, the See also:

• EQUIVALENT

equivalent focus can be made very great in comparison with the length of the See also:

• TUBE (Lat. tuba)

tube . This permits the employment of a spectroscope furnished with a collimator of long focus . The magnifying power is computed by the same See also:

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

formula as in the case of the Gregorian telescope . The Newtonian telescope is represented in Fig . 8 . A A is a See also:

• CON

con-See also:

• CAVE (Lat. cavea, from caves, hollow)
• CAVE, EDWARD (1691–1754)
• CAVE, WILLIAM (1637–1713)

cave mirror whose axis is a a .

Parallel rays falling on A A converge on the plane mirror B B, and are thence reflected at New right angles to the axis, forming an image in the focus of toalea. the eye-piece E . The See also:

• SURFACE

surface of the large mirror should be a paraboloid of revolution, that of the small mirror a true optical plane . The magnifying power is = Fie . This form is employed in the construction of most modern reflecting telescopes . A glass prism of See also:

• TOTAL

total reflection is sometimes substituted for the plane mirror . The Herschelian or front view reflector is represented in fig . 9 . A A is a concave parabolic mirror, whose axis a c is inclined to the axis of the tube a b so that the image of an object in the focus of the mirror may be viewed by an eye-piece at E, the angle b a c being Her- equal to the angle c a E . This form was adopted by the Her- n. See also:

• ELDER (0. Eng. ellarn; Ger. Holunder; Fr. sureau)
• ELDER (Gr. 1rpev(3iTepos)

elder Herschel to avoid the loss of light from reflection in scheila the small mirror of the Newtonian telescope . The front view telescope, however, has hardly been at all employed except by the Herschels . But at the same time none but the Herschels have swept the whole See also:

• SKY (M. Eng. skie, cloud; O. Eng. skua, shade; connected with an Indo-European root sku, cover, whence " scum," Lat. obscurus, dark, &c.)

sky for the See also:

• DISCOVERY

discovery of faint nebulae; and A probably no other astronomers have worked for so many See also:

• HOURS, CANONICAL

hours on end for so many nights as they did, and they emphasize the easy position of the observer in using this form of instrument . Construction of Specula .

The See also:

• COMPOSITION
• COMPOSITION (Lat. compositio, from componere, to put together)

composition of metallic specula in the present day differs very little from that used by Sir See also:

• ISAAC (Hebrew for " he laughs," on explanatory references to the name, see ABRAHAM)

Isaac See also:

• NEWTON
• NEWTON, ALFRED (1829–1907)
• NEWTON, JOHN (1725-1807)
• NEWTON, JOHN (1823-1895)
• NEWTON, SIR CHARLES THOMAS (1816-1894)
• NEWTON, SIR ISAAC (1642-1727)

Newton . Many different See also:

• ALLOYS (through the Fr. aloyer, from Lat. alligare, to combine)

alloys have been suggested, some including See also:

• SILVER

silver, See also:

• NICKEL
• NICKEL (symbol Ni, atomic weight 58.68 (0=16))

nickel, See also:

• ZINC

zinc or See also:

• ARSENIC (symbol As, atomic weight 75.0)

arsenic; but that which has practically been found best is an alloy of four equivalents of See also:

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

copper to one of See also:

• TIN (Lat.- stannum, whence the chemical symbol " Sn "; atomic weight =117.6, 0=16)

tin, or the following See also:

• PRO

pro-portions by See also:

• WEIGHT

weight: copper 252, tin 117.8 . Such speculum See also:

• METAL
• METAL (through Fr. from Lat. metallum, mine, quarry, adapted from Gr. µATaXAov, in the same sense, probably connected with ,ueraAAdv, to search after, explore, µeTa, after, aAAos, other)

metal is exceedingly hard and brittle, takes a See also:

• FINE

fine white See also:

• POLISH

polish, and when protected from See also:

• DAMP

damp has little liability to tarnish . The See also:

• PROCESS

process of casting and annealing, in the case of the specula of the great Melbourne telescope, was admirably described by Dr See also:

• ROBINSON, EDWARD (1794–1863)
• ROBINSON, HENRY CRABB (1777–1867)
• ROBINSON, JOHN (1575–1625)
• ROBINSON, JOHN (1650-1723)
• ROBINSON, JOHN THOMAS ROMNEY (1792–1882)
• ROBINSON, MARY [" Perdita "] (1758–1800)
• ROBINSON, SIR JOHN BEVERLEY, BART
• ROBINSON, SIR JOSEPH BENJAMIN (1845– )
• ROBINSON, THEODORE (1852-1896)

Robinson in Phil . Trans., 1869, 159, p . 135 . Shaping, polishing and figuring of specula are accomplished by methods and tools very similar to those employed in the construction of lenses . The reflecting surface is first ground to a spherical form, the parabolic figure being given in the final process by regulating the size of the See also:

• PITCH
• PITCH, MUSICAL

pitch squares and the stroke of the polishing See also:

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

machine . Soon after See also:

• LIEBIG, JUSTUS VON, BARON (1803-1873)

Liebig's discovery of a process for depositing a film of pure metallic silver upon glass from 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 of silver in See also:

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

solution, Steinheil (Gaz . Univ. d'See also:

• AUGSBURG
• AUGSBURG, CONFESSION OF
• AUGSBURG, WAR OF THE LEAGUE OF

Augsburg, 24th See also:

• MARCH
• MARCH (1) (from Fr. marcher, to walk; the earliest sense in French appears to be " to trample," and the origin has usually been found in the Lat. marcus, hammer; Low Lat. marcare, to hammer; hence to beat the road with the regular tread of a soldier: cf.
• MARCH, AUZIAS (c. "1395-1458)
• MARCH, EARLS OF
• MARCH, FRANCIS ANDREW (1825– )

March 1856), and later, in-dependently, See also:

• FOUCAULT, JEAN BERNARD LEON (1819—1868)

Foucault (Comptes Rend us, vol. xliv., See also:

• FEBRUARY

February 1857), proposed to employ glass for the specula of telescopes, the reflect-mg surface of the glass speculum to be covered with silver by Liebig's process . Those silver-on-glass specula are now the rivals of the achromatic telescope, and it is not probable that many telescopes with metal specula will be made in the future . The best speculum metal and the greatest care are no See also:

• GUARANTEE (sometimes spelt " guarantie " or " guaranty "; an O. Fr. form of " warrant," from the Teutonic word which appears in German as wahren, to defend or make safe and binding)

guarantee of freedom from tarnish, and, if such a mirror is much exposed, as it must be in the hands of an active observer, frequent repolishing will probably be necessary .

This involves refiguring, which is the most delicate and costly process of all . Every time, therefore, that a speculum is repolished, the future quality of the instrument is at stake; its focal length will probably be altered, and thus the value of the constants of the See also:

• MICROMETER (from Gr. wcp6c, small, jArpov, a measure)

micrometer also have to be redetermined . Partly for these reasons the reflecting telescope with metallic mirror has never been a favourite with the professional astronomer, and has found little employment out of See also:

• ENGLAND
• ENGLAND, THE CHURCH OF

England.' In England, in the hands of the Herschels, See also:

• ROSSE, EARL OF
• ROSSE, WILLIAM PARSONS, 3RD EARL

Rosse, Lassell and De la See also:

• RUE (Fr. rue, Lat. ruta, from Gr. pur)

Rue it has done ' There is a noteworthy exception in the case of the 18-in. speculum-metal mirror employed by Sir William See also:

• HUGGINS, SIR WILLIAM (1824-1910)

Huggins at 'See also:

• PULSE

Pulse Hill, with which a large part of his remarkable and important See also:

• SERIES (a Latin word from serere, to join)

series of astrospectroscopic results have been obtained . So far as we know, this mirror has never been repolished since its first See also:

• INSTALLATION

installation in 1870, and still retains its admirable surface . One of Short's mirrors, made about 176o or 1770, of 6-in. aperture, now in the See also:

• POSSESSION
• POSSESSION (Lat. possessio, possidere, to possess)

possession of Sir William Huggins, has surfaces which still retain their See also:

• ORIGINAL

original perfection although they have never been repolished.splendid service, but in all these cases the astronomer and the instrument-maker were one . The silver-on-glass mirror has the enormous advantage that it can be resilvered with little trouble, at small expense, and without danger of changing the figure . Glass is lighter, stiffer, less costly and easier to work than speculum metal . Silvered mirrors have also some advantage in light grasp over those of speculum metal, though, aperture for aperture, the former are inferior to the modern object-glass . Comparisons of light grasp derived from small, fresh, carefully silvered surfaces are sometimes given which See also:

• LEAD
• LEAD (pronounced iced)

lead to illusory results, and from such experiments Foucault claimed superiority for the silvered speculum over the object-glass . But Sir See also:

• DAVID
• DAVID (a Hebrew name meaning probably beloved 1)
• DAVID, FELICIEN (1810—1876)
• DAVID, GERARD [GHEERAERT DAVIT], (?=1523)
• DAVID, PIERRE JEAN (1789–1856)
• DAVID, ST (Dewi, Sant)

David Gill found from experience and careful comparison that a silvered mirror of 12-in. aperture, mounted as a Newtonian telescope (with a silvered plane for the small mirror), when the surfaces are in fair See also:

• AVERAGE

average condition, is equal in light grasp to a first-rate refractor of to-in. aperture, or See also:

• AREA

area for area as 2: 3 . This ratio will become more equal for larger sizes on account of the additional thickness of larger object-glasses and the consequent additional absorption of light in transmission . Mounting of Telescopes .

The proper mounting of a telescope is hardly of less importance than its optical perfection . Freedom from tremor, ease and delicacy of See also:

• MOVEMENT

movement and facility of directing the instrument to any desired object in the heavens are the primary qualifications . Where accurate See also:

• DIFFERENTIAL

differential observations or photographs involving other than instantaneous exposures have to be made, the additional condition is required that the optical axis of the telescope shall accurately and automatically follow the object under observation in spite of the apparent diurnal See also:

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

motion of the heavens, or in some cases even of the apparent motion of the object relative to neighbouring fixed stars . Our limits forbid a See also:

• HISTORICAL

historical account of the earlier endeavours to fulfil these ends by means of motions in See also:

• ALTITUDE (Lat. altitudo, from altus, high)

altitude and See also:

• AZIMUTH (from the Arabic)

azimuth, nor can we do more than refer to mountings such as those employed by the Herschels or those designed by 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 Rosse to over-come the See also:

• ENGINEERING

engineering difficulties of mounting his huge telescope of 6 ft. aperture . Both are abundantly illustrated in most popular See also:

• WORKS

works on See also:

• ASTRONOMY (from Gr. &arpov, a star, and v ,usiv, to classify or arrange)

astronomy, and it seems sufficient to refer the reader to the original descriptions.2 We pass, therefore, directly to the See also:

• EQUATORIAL

equatorial telescope, the instrument See also:

• PAR (Lat. par, equal)

par excellence of the modern extra-See also:

• MERIDIAN
• MERIDIAN (from the Lat. meridianus, pertaining to the south or mid-day)

meridian astronomer . The See also:

• ARTICLE (from Lat. articulus, a joint)

article TRANSIT CIRCLE describes one form of mounting in which the telescope is simply a refined substitute for the See also:

• SIGHTS

sights or pinules of the old astronomers . The present article contains a description of the mounting of the various forms of the so-called See also:

• ZENITH (from the Arabic)

zenith telescope . In its simplest form the mounting of an equatorial telescope consists of an axis parallel to the See also:

• EARTH
• EARTH (a word common to Teutonic languages, cf. Ger. Erde, Dutch aarde, Swed. and Dan. jord; outside Teutonic it appears only in the Gr. 'pq'e, on the ground; it has been connected by some etymologists with the Aryan root ar-, to plough, which is seen in
• EARTH, FIGURE OF
• EARTH, FIGURE OF THE

earth's axis, called "the polar axis"; a second axis at right angles to the polar axis called " the See also:

• DECLINATION (from Lat. declinare, to decline)

declination axis "; and the telescope tube fixed at right angles to the declination axis . In Fig. so A A is the polar axis; the telescope is attached to the end of the declination axis; the latter rotates in See also:

• BEARINGS

bearings which are attached to the polar axis and concealed by the telescope itself . The telescope is counterpoised by a weight attached to the opposite end of the declination axis . The lower See also:

• PIVOT (Fr. pivot; probably connected with Ital. pivolo, peg, pin, diminutive of piva, pipa, pipe)

pivot of the polar axis rests in a See also:

• CUP (in O.E. cuppe; generally taken to be from Late Lat. cuppa, a variant of Lat. cupa, a cask, cf. Gr. KuaeXXov)

cup-bearing at C, the upper bearing upon a strong metal casting M M attached toa See also:

• STONE
• STONE (0. Eng. shin; the word is common to Teutonic languages, cf. Ger. Stein, Du. steen, Dan. and Swed. sten; the root is also seen in Gr. aria, pebble)
• STONE, CHARLES POMEROY (1824-1887)
• STONE, EDWARD JAMES (1831-1897)
• STONE, FRANK (1800-1859)
• STONE, GEORGE (1708—1764)
• STONE, LUCY [BLACKWELL] (1818-1893)
• STONE, MARCUS (184o— )
• STONE, NICHOLAS (1586-1647)

stone See also:

• PIER (older forms per or Pere, from Med. Lat. pera; the word for piers out of water. For river piers, where a firm, watertight is of obscure origin, and the connexion with Fr. Pierre, Lat. stratum is found at a moderate depth below the river-bed, the pctr

pier S . A See also:

• VERTICAL (from Lat. vertex, highest point)

vertical plane passing through A A is therefore in the meridian, and the polar axis is inclined to the See also:

• HORIZON (Gr. 6pfTwv, dividing)

horizon at an angle equal to that of the See also:

• LATITUDE (Lat. latitudo, latus, broad)

latitude of the place of observation .

Thus, when the declination axis is See also:

• HORIZONTAL

horizontal the telescope moves in the plane of the meridian by rotation on the declination axis only . Now, if a graduated circle B B is attached to the declination axis, together with the necessary verniers or microscopes V V for See also:

• READING

reading it (see TRANSIT CIRCLE), so arranged that when the telescope is turned on the declination axis till its optical axis is parallel to A A the See also:

• VERNIER, PIERRE (c. 1580–1637)

vernier reads 0° and when at right angles to A A 90°, then we can employ the readings of 2 Herschel, Phil . Trans., 1795, 85, p . 347; Rosse, Phil . Trans_ 184o, p . 503; 1861, p . 681 . A this circle to measure the polar distance of any star seen in the telescope, and these readings will also be true (apart from the effects of atmospheric See also:

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

refraction) if we rotate the instrument through any angle on the axis A A . Thus one important attribute of an equatorially mounted telescope that, if it is directed to any fixed star, it will follow the diurnal motion of that star from rising to setting by rotation of the polar axis only . If we now attach to the polar axis a graduated circle D D, called the " See also:

• HOUR

hour circle," of which the microscope or vernier R reads on when the declination axis is horizontal, we can obviously read off the hour angle from the meridian of any star to which the telescope may be directed at the instant of observation . If the See also:

• LOCAL

local sidereal time of the observation is known, the right See also:

• ASCENSION
• ASCENSION, FEAST OF THE

ascension of the star becomes known by adding the observed hour angle to the sidereal time if the star is See also:

• WEST
• WEST, BENJAMIN (1738-1820)
• WEST, NICHOLAS (1461-1533)

west of the meridian, or subtracting it if See also:

• EAST
• EAST, ALFRED (1849- )

east of the meridian . Since the transit circle is preferable to the equatorial for such observations wherein great accuracy is required, the declination and hour circles .3f an equatorial are employed, not for the determination of the right ascensions and declinations of See also:

• CELESTIAL, OR STELLAR

celestial objects, but for directing the telescope with ease and certainty to any object situated in an approximately known position, and which may or may not be visible to the naked eye, or to define approximately the position of an unknown object .

Further, by causing the hour circle, and with it the polar axis, to rotate by clockwork or some equivalent See also:

• MECHANICAL

mechanical contrivance, at the same angular velocity as the earth on its axis, but in the opposite direction, the telescope will, apart from the effects of refraction, automatically follow a star from rising to setting . Types of Equatorials.—Equatorial mountings may be divided into six types . (A) The pivots or bearings of the polar axis are placed at its extremities . The declination axis rests on bearings attached to opposite sides of the polar axis . The telescope is attached to one end of the declination axis, and counterpoised by a weight at the other end, as in fig. io . (B) The polar axis is supported as in type A; the telescope is placed between the bearings of the declination axis and is mounted symmetrically with respect to the polar axis ; no counterpoise is therefore requisite . (C) The declination axis is mounted on the prolongation of the upper pivot of the polar axis; the telescope is placed at one end of the declination axis and See also:

• COUNTER

counter-poised by a weight at the other end . (D) The declination axis is mounted on a forked piece or other similar contrivance attached to a prolongation of the upper pivot of the polar axis; the telescope is mounted between the pivots of the declination axis . (E) The eye-piece of the telescope is placed in the pivot of the polar axis; a portion or the whole of the axis of the telescope tube coincides with the polar axis . (F) The telescope is fixed and the rays are reflected along its axis from an See also:

• EXTERNAL

external mirror or mirrors . Mountings of types A and B—that is, with a long polar axis sup-ported at both ends—are often called the " See also:

• ENGLISH

English mounting," and type C, in which the declination axis is placed on the extension of the upper pivot of the polar axis, is called the " See also:

• GERMAN
• GERMAN, DUTCH AND SCANDINAVIAN

German mounting," from the first employment of type C by Fraunhofer . A description of some of the best examples of each type will illustrate their relative advantages or peculiarities .

Type A.—Fig. io may be taken as a practical example of the earlier equatorials as made by See also:

• TROUGHTON, EDWARD (1753-1835)

Troughton in England and afterwards by Gambey for various See also:

• CONTINENTAL

Continental observatories . In the Phil . Trans . for 1824 (part 3, pp . 1–412) will be found a description by Sir John Herschel and Sir See also:

• JAMES
• JAMES (Gr. 'IlrKw,l3or, the Heb. Ya`akob or Jacob)
• JAMES (JAMES FRANCIS EDWARD STUART) (1688-1766)
• JAMES, 2ND EARL OF DOUGLAS AND MAR(c. 1358–1388)
• JAMES, DAVID (1839-1893)
• JAMES, EPISTLE OF
• JAMES, GEORGE PAYNE RAINSFOP
• JAMES, HENRY (1843— )
• JAMES, JOHN ANGELL (1785-1859)
• JAMES, THOMAS (c. 1573–1629)
• JAMES, WILLIAM (1842–1910)
• JAMES, WILLIAM (d. 1827)

James See also:

• SOUTH
• SOUTH, ROBERT (1634–1716)

South of the equatorial telescope which they employed in their measurements of ~.s double stars . The polar axis was similar in shape to that of fig. lo *F' f91Al~llli~SN,, g• and was composed of sheets of tinned See also:

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

iron . In See also:

• SMYTH (or SMITH), JOHN (c. 1570-1612)
• SMYTH (or SMITH), WILLIAM (c. 1460-1514)
• SMYTH, CHARLES
• SMYTH, SIR WARINGTON WILKINSON (1817-1890)

Smyth's celebrated ` ` 71,1 d" f7ilRN lil See also:

• BEDFORD

Bedford telescope the polar axis was ,i'OrSh, fi ;i I of See also:

• MAHOGANY

mahogany . Probably the best l innsv example of this type of mounting - ,,, .. , _ T applied to a refractor is that made F1c. i 1.-Melbourne Reflector. by the elder Cooke of See also:

• YORK
• YORK (HOUSE OF)
• YORK, EDMUND OF LANGLEY, DUKE OF (1341-1402)
• YORK, EDWARD
• YORK, FREDERICK AUGUSTUS, DUKE OF (1763-1827)
• YORK, RICHARD, DUKE

York for Flet- See also:

• CHER

cher of Tarnbank; the polar axis is of See also:

• CAST (from the verb meaning " to throw "; the word is Scand. in origin, cf. Dan. kaste, and Swed. kasta; " cast " in Middle Eng. took the place of the A.S. weor pan, cf. Ger. werf en)

cast iron and the mounting very satisfactory and convenient, but unfortunately no detailed description has been published . In recent Great years no noteworthy refractors have been mounted on this See also:

• PLAN
• PLAN (from Lat. planes, flat)

plan; but type A has been chosen by Grubb for the Mel- great Melbourne reflector, of 48-in. aperture, with marked See also:

• BOURNE (southern form of burn, Teutonic born, bran, burna)
• BOURNE, or BOURN
• BOURNE, VINCENT (1695-1747)

bourne ingenuity of adaptation to the peculiar requirements telescope. of the case . Fig. i i shows the whole instrument on a small See also:

• SCALE
• SCALE (1) A

scale with the telescope directed to the See also:

• POLE (FAMILY)
• POLE, REGINALD (1500-1558)
• POLE, RICHARD DE LA (d. 1525)
• POLE, WILLIAM (1814—1900)

pole, and the hour circle set 6^ from the meridian . Type B.—The most important examples of type B are See also:

• AIRY, SIR GEORGE BIDDELL (1801-1892)

Airy's equatorial at See also:

• GREENWICH

Greenwich (originally made to carry a telescope of aperture, but now fitted with a telescope by Grubb of 28-in. aperture), and the photographic equatorials of 13-in. aperture employed at 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 other See also:

• FRENCH
• FRENCH, DANIEL CHESTER (1850– )
• FRENCH, NICHOLAS (1604-1678)

French observatories, of which the object-glasses were made by the See also:

• BROTHERS, RICHARD (1757-1824)

brothers See also:

• HENRY
• HENRY (1129-1195)
• HENRY (c. 1108-1139)
• HENRY (c. 1174–1216)
• HENRY (Fr. Henri; Span. Enrique; Ger. Heinrich; Mid. H. Ger. Heinrich and Heimrich; O.H.G. Haimi- or Heimirih, i.e. " prince, or chief of the house," from O.H.G. heim, the Eng. home, and rih, Goth. reiks; compare Lat. rex " king "—" rich," therefore " mig
• HENRY, EDWARD LAMSON (1841– )
• HENRY, JAMES (1798-1876)
• HENRY, JOSEPH (1797-1878)
• HENRY, MATTHEW (1662-1714)
• HENRY, PATRICK (1736–1799)
• HENRY, PRINCE OF BATTENBERG (1858-1896)
• HENRY, ROBERT (1718-1790)
• HENRY, VICTOR (1850– )
• HENRY, WILLIAM (1795-1836)

Henry and the mountings of any importance to be provided with clockwork .

The instrument, by See also:

• GAUTIER
• GAUTIER, EMILE THEODORE LEON (1832-1897)
• GAUTIER, THEOPHILE (1811-1872)

Gautier of Paris. shown in fig . 13, is described in detail by See also:

• STRUVE, FRIEDRICH GEORG WILHELM (1793-1864)

Struve (Beschreibung See also:

• DES

des These instruments have done admirable work in connexion with auf der Sternwarte zu Dorpat befndlichen grossen Refractors von the great See also:

• INTERNATIONAL, THE

international undertaking, the See also:

• CARTE, THOMAS (1686-1754)

Carte du Ciel . The See also:

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

general construction will be understood from fig . 12 . The double polar axis is composed of hollow metal beams of triangular See also:

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

section . The hour circle has two toothed circles cut upon it, one acted upon by a See also:

• WORM

worm screw mounted on the pier and driven by clockwork, the other by a second worm screw attached to the polar axis, which can be turned by a handle in the observer's See also:

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

hand and thus a slow movement can be given to the telescope in right ascension irides After an See also:

• ILLUSTRATION

illustration in La Nature, by permission of See also:

• MASSON, DAVID (1822–1907)
• MASSON, LOUIS CLAUDE FREDERIC (1847– )

Masson et Cie . pendently of the See also:

• CLOCK

clock . Slow motion in declination can be communicated by a screw acting on a long arm, which latter can be clamped at See also:

• PLEASURE (through Fr. plaisir from Lat. placere, to please; Gr. rlbovii)

pleasure to the polar axis . An oblong metallic See also:

• BOX
• BOX (Gr. irl or, Lat. buxus, box-wood; cf. 2r6Eir, a pyx)

box fitted with pivots, whose bearings are attached to the triangular beams, forms the tube for two parallel telescopes; these are separated throughout their length by a metallic See also:

• DIAPHRAGM (Gr. Scat/pay,aa, a partition)

diaphragm . The chromatic aberration of the object-glass of one of these telescopes is corrected for photographic rays, and the image formed by it is received on a highly sensitive photographic plate . The other telescope is corrected for visual rays and its image is formed on the plane of the spider-lines of a filar micrometer . The peculiar form of the tube is eminently suited for rigid preservation of the relative See also: