TELESCOPES, BY BOSS, WRAY, BURON, Arc.
[Class X.
264
■through the central aperture, it follows that with a suf-ficient field of view, we cannot have as low a power aswould be desirable, without increasing the central aperture Iand the small reflector so much as to injure the telescopefor the reception of high astronomical powers. To 'obviate this, if possible, Yarley and Son have, in the firstinstance, made the great speculum of the shortest eligible ifocus, by which means the power of the small speculum jis lessened and brought nearer, the angle of view beingincreased in the same degree. The small speculums aremounted in tubes of any length less than double theirfocus. This arrangement gives a more effective darkmargin around the pencils of light, and such as wouldrequire a larger disc if placed behind the speculum. Smallspeculums of longer focus may thus be used with nogi eater obstruction of light, and an equally good fieldretained with a lower power. The mounting the smallspeculums in tubes effectually secures them from injurywhilst in place, and when removed, a small cap com-pletely excludes them from the air. The arm whichsupports them remains in the tube, it having a concentricring, into which the small tubes are screwed for use, andfrom which they a!*e more easily detached than whenaffixed to separate slides in the common way.
Boss (No. 254, pp. 435, 436) exhibits a telescope of3 feet focal length and inches aperture, of English flint glass, which, examined on test objects at 150 yards(consisting of two black marble balls, highly polished,placed in full sunshine; a watch-dial, and small balls ofwhite ivory on a black ground), was found to performwell, giving well-concentrated images of the artificialstars produced on the marble balls, with but a small traceof uncorrected colour.
A Council Medal was awarded to Mr. Boss for thistelescope in connexion with microscopes.
Callaghan (No. 268, p. 437) exhibits a telescope in-tended for use in deer-stalking.
Salmon (No. 266, p. 437) exhibits several day andnight telescopes, intended for ships’ use, which are goodfor their price.
Richardson (Xo. 264, p. 437) exhibits a small reflect-ing telescope, of 3 feet 8 inches in length. Tuo largereflector, made of one piece of solid crown glass, wascither painted, or had paper pasted against its back, there-by giving it that tendency to change figure by heat,against which Sir John Iierschcl has so strongly cautionedall constructors of glass mirrors.
Boyle (No. 392, p. 451) exhibits a reflecting telescope,intended for use without a tube.
Wray (No. 309, pp. 442, 443) exhibits a 7-feet refract-ing telescope, with 44 inches aperture. The peculiarityof this telescope is the substitution of a solid substanceinstead of flint glass. (See Illustrated Catalogue.)
On trial it was found to be badly achromatised. Thecolour above the image did not seem to be that usuallycalled the secondary spectrum, but a remain of colournot fully compensated. The glass was neither fullycorrected for sphericity nor for colour. It was observedthat the object lens had rather a strong yellow tint, audwas somewhat blotchy, as if the material used was notquite uniform in colour; all interior reflections weredestroyed, so that it could not be suspected to be otherthan a single glass. As a telescope it is not very good:but, though an imperfect trial, it is yet a fair attempt tomove out of the beaten track, aud as a step towards thepossible revival of fluid or semi-fluid object glasses,deserves commendation.
Watkins and IIill (No. 659, p. 466*) exhibit a tele-scope 3^ feet focal length, and diameter of object glass2} inches, furnished with a finder, vertical and horizontalrack-work motion, and cye-pieces with powers to 220.
Marratt No. 409, p. 454) sends an achromatic tele-scope of 5 feet focal length; it is furnished with powersof 65, 85, 120, 200, and 2S0. On trial it was found to begood, and to deserve Honourable Mention.
Harris and Son (No. 149, pp. 428, 429) have a micro-metrical and double image telescope and “ coming-upglass,” for measuring distances either on land or sea.
This instrument is designed for the purpose of ascer-taining the distance and dimensions of any inaccessible
object by means of simple calculation; also to determine,without* calculation, the distance of any known object,by means of a set of tables adapted to the scale. It isintended to act as a micrometer for the purposes of astro-nomy, and as a “ coming-up glass,” to ascertain whethera ship be approaching to the observer or receding fromhim, and withal to combine simplicity of construction.
Huron (France , No. 443, p. 1199) exhibits a telescope,the object glass of which is of rock crystal, 4 feet2 inches in diameter, and 6 feet 3 inches focal length.Attached to the telescope is a finder, which embraces afield of view from 5° to 6°, and lias cross wires, which,owing to the great illumination of the field, may be seenduring the darkest night, and consequently the starbrought into the centre of the field. On examining thisinstrument it was found to be good in every respect. Itis fixed upon a very steady cast-iron stand, furnishedwith three small castors, brought into operation by meansof rack-work when necessary to remove the instrument.
The object glass of this telescope is of rock crystal,which requires great care in its prepartion on account ofits property of double refraction. In its working thefollowing particulars are necessary to be attended to:—
1 . The crystal must be cut perpendicular to its axis.
2. In working the axis, the spherical surface mustalways coincide with the axis of crystallization.
3. The curvature of their surfaces must not be made toolarge in the angles, as double refraction would then bevisible to the eye.
M. Biot has shown that an angle of 5" must alwaysexist, but that this is not visible to the eye.*
The formula of Huygens has been used in workingthe rock-crystal object-glass; it is as follows:—Sup-posing that "the refractions, both ordinary and extraor-dinary, take place in the plane of the principal section.
ni 2 . vi"~ (1 + a 2 )m' 2 . a 2 w?" 2 ’in which m' = The index of refraction.
in = The ordinary index, or minimum.m" = The extraordinary index or maximum*a = The tangent of the angle of ordinationof the luminous rays with the axis ofthe lens.
Thus in' is always an index of refraction, intermediateto in and m".
M. Huron has also exhibited another telescope of aboutthe same dimensions, which was found to be good. 11 issupported upon a stand, invented by the late M. Cauehois,tlie appearance of which is elegant, but as it compels theobserver to stand during the time of observation, and asit is necessary to be moved entirely when large azimuthalangles have to be passed over, it is less coincident thanthe east-iron stand before described. It is made of wood,which, though rendering it liable to be affected by varia-tions of moisture, gives it the advantage of being lighterthan one constructed of iron.
M. Huron has also exhibited telescopes of various sizes,provided with terrestrial aud celestial eye-pieces, audmounted upon brass stands: also nautical and pockettelescopes. In the construction of his eye-pieces, M.Huron pays strict attention to the rules of M. Biot.
Many of the portable telescopes were tried, and theirperformance was found to be very good; they are re-markably cheap.
A Council Medal was awarded to M. Burcn.
Lebrun (France , No. 298, p. 1191) exhibits severalachromatic telescopes of a very good kind, and which areremarkably cheap.
Kinzelbach (Wurtemburg, No. 26, p. 1115) has ex-hibited an achromatic telescope of about 24 inches aper-ture, and 234 inches focus, constructed on the dialytieprinciple, in which the correction of the dispersion of thecrown lens is performed by a fliut lens of only half theaperture, placed mid-way between the crown lens and the
* See Biot. “ Traite dWstronomie Physique ” vol. ii.,1844, and vol. iii., published in 1846 ; also Memoires surles Oculaires Multiplies et Achromatiques, preseutes aI'Academic des Sciences, 1843.”