Class X.]
OBJECT AXD OTHER GLASSES FOB TELESCOPES.
2CS
] *8 inch in diameter. This is a most elaborate work,and hears examination well by high powers.
This invention, apart from its great ingenuity and theperseverance by which it has been originated, promises tobe of great utility in reduciug standard works with accu-racy only exceeded by a degree of minuteness calculatedto render any attempt at counterfeit next to impossible.
Object Glasses for Telescopes .
Simms (No. 741, pp. 475*, 47G*, 477*) exhibits severalachromatic object-glasses; viz., one of 9 inches aperture,two of 8 inches aperture, one of G*9 inches aperture, andone of 4 incites aperture. The largest which is exhibitedis entirely of English manufacture, and the discs, botli offlint and crown, were made by Messrs. Chance, of Bir-mingham . In all the remaining glasses, the crown is ofEnglish —and the flint of foreign—manufacture. Theobject-glasses are altogether the work of the exhibitor,and on examination were found to be good.
It is observable that the crown glass in all these speci-mens has a greenish hue, and Mr. Simms states, withreference to this particular, that he has found from expe-rience (as might be expected from the extinction of oneextreme, and consequent shortening of the total spec-trum), that there is less irrationality between this kind ofcrown glass and the denser kind of flint, than betweenthe same specimens of flint and any other crown or plateglass that he has been able to procure. "We understandMr. Simms’ practice in determining the corrections re-quired for achromatism consists in using that part of thespectrum which is included between the lines B and G;he finds, however, differences of small amount, sufficientto produce a sensible effect upon the results, betweendiscs of glass sent to him at the same time, and which hesupposes are prepared, if not at the same time, at least ofthe same materials.
The following are the specific gravities, See. of speci-mens of flint and crown glass manufactured by Messrs.Chance and Co., of Birmingham :—
Kind of Glass.
Specific
Gravity.
Index ofRefractionG.
Dispersive
I'ovver
B. to G.
Flint - -
3-5S3
1-C10D
1-G33G
0-030
Crown - -
2-539
1-5123
1*5274
0*023
To Mr. Simms the Jury consider very great merit isdue, not only for the exquisite workmanship displayed inevery instrument exhibited by him, but also for theseveral new inventions and arrangements which, by thegreater facility they afford to observation, cannot fail inthe advancement of astronomy. The Jury unanimouslyvoted a Council Medal to Mr. Simms, which award waspassed unanimously by the Group; but, unfortunately,was not confirmed by the Council of Chairmen ; conse-quently the medal which Mr. Simms will receive is notthat which, in the opinion of the Jury, is due to him.
Goddard (No. 274, p. 43S) exhibits an object-glass.
Buro.n (France, No. 443, p. 1199) has exhibited auachromatic glass 7'5 inches diameter and 10*8 feet focallength; the crown and flint glass for which were madeby the late M. Gurnard, of Paris , and the curves accord-ing to the theory of Sir John Ilerschel, and is designedto show stars of the eleventh magnitude. It was nottried by the Jury on account of its tube being delayedat the Custom-house under a very heavy charge.
Solid Eyc-picces.
Rkade (No. 254a, p. 43G) exhibits two solid eye-pieces.They consist of three lenses, the anterior and posteriorbeing double convex of crown glass, and the intermediateone double concave of flint. The contact surfaces arecemented together, and consequently the action upon therays of light is similar to that of a single lens. They areapplicable to all astronomical instruments, micrometers,and microscopes, and also as a general magnifying power.
The novelty of the solid eye-piece consists in its construc-tion, by which a large and flat field of view is obtained,together with the removal of spherical and chromaticaberration. These advantages are obtained by means ofthe great thickness of the flint lens, which is a perfectlynew feature in the construction of eye-pieces.
Sir David Brewster ’s achromatic sphere which he pro-poses to use as an object glass for the microscope, approxi-mates to this form, but the field is not fiat. Thegeneral mathematical expressions for the curves of thesolid eye-piece have not as yet been fully worked out;but it may be stated as a rule for the practical optician,that the radii of the outer curves of the two convexlenses, must be in the proportion of the dispersive ratio ofthe crown and flint glass employed on each eye-piece, andthe inner contact surface must be such as by experimentwill perfect the achromatism. The solid eye-pieces ex-hibited have focal lengths of u*5 inches and U’75 inches,and it happens that while the eye-piece of shorter focallength is barely brought up to a flat field of view, the oneof longer focal length is carried rather beyond, and istherefore slightly over corrected. These small ei-rors areupon the whole satisfactory, inasmuch as they show thata truly flat field is attainable.
Sir David Brewster observes in his “ Treatise onOptics,” that “achromatic eye-pieces, where one lens onlyis wauted, may be composed of two or three lenses, ex-actly on the same principle as object-glasses; such, in-deed, as the cemented triple object-glass made for themicroscope, which was described by the Rev. J. B. Readesome years ago, in a paper read before the Royal Society ,as being an efficient eye-piece for telescopes;—in fact,the then best known form of a vertical single lens; but,the smallness of the field is an insuperable objection to itsuse with a micrometer. The larger field of view nowobtained in the solid eye-piece is solely due to the thick-ness of the flint, which causes the image of the object-glass, where the eye is placed, to be formed so near theeye-piece that its diameter is seen under a considerableangle; and it is found that in the lower powers thediameter of the field of view is larger than in the ordinarynegative eye-piece.”
The positive eye-piece almost universally used, isknown as Ramsden’s: the field is fiat, which is requisitefor a measuring instrument, but it is far from being freefrom colour.
The Astronomer Royal states* that the positive eye-piece at present used is not achromatic, and cannot hemade so ; and considerable inconvenience is occasioned bythis defect, for when the object is not far from the centreof the field, the chromatic confusion is much greater thanthat produced by spherical aberration; since it varies asthe distance from the centre, and in this confusion thereis nothing to point out the centre of the coloured line.
The new solid eye-piece is therefore looked upon as animportant addition to the working apparatus of an astro-nomer, inasmuch as the great evil of chromatic confusion,which prevents any accurate measurements at the ex-tremities of the field, does not here exist at all, but thewebs of the micrometer, though ever so distant, may beseen as fine black lines. The field of view is also freefrom illumination by false light; no light is lost, as in theusual construction by inner reflection from the surfacesof the lenses,—and there is no formation of the falseimage or ghost of planets, nor of the bright stars, whichinjures the best part of the field.
An examination of this cye-piece, as applied to atelescope, in comparison with the action of the ordinaryeye-pieces applied to the same telescope, made by Mr.Glaisher , extending over a period of three hours, fullyimpressed him with its superiority; the field of view wasblack, and the stars appeared as very bright points. Inconsequence of the purity of the achromatism it willprobably be found that in determining the colours of fixedstars, these eye-pieces may be of essential service. TheJury awarded Mr. Reade the Prize Medal.
* See “Cambridge Philosophical Transactions,” vol. iii.p. 56.