270

OPTICAL GLASSES—EXAMINATION OF CHANCE’S DISC.

[Class X.

Optical Glass .

The disc, of 29 inches in diameter, exhibited by Mr.Chance (p.477*J having undergone a partial examinationby the Jury, it was considered that a satisfactory opinioncould not be formed of it, owing to the irregularities inits surfaces, until it had undergone the operation ofpolishing, so as to give it a plane, or nearly plane figure.This operation having been performed by Mr. Boss, andthe disc being reported by him fit for further examina-tion, it was agreed to subject it to such on Tuesday, the9th of September 1851, for which day (it being consideredby the Committee desirable to have the attendance of asmany of the Jury as could be brought together) a Jurysummons ■was issued, which was responded to by Profes-sors Potter and Colladon , who attended with the Com-mittee, consisting of Sir J. Herschel, Mr. Glaisher , andProfessor Miller. Mr. Boss, Mr. Simms, and Mr. ljontems(the latter on the part of Mr. Chance) were also presentat the examination.

The dimensions of the disc were as follows:—

Largest diameter . . . 29^ inches.

Shortest ,, ... 29 ,,

Thickuess, from . , . 2*2 to 2*25 inches.

Its weight was stated to be somewhere about 200 lbs.The specific gravity about 3*56 .... 3-58. The surfaceswere highly polished and brought to a figure veryslightly deviating from true planes, so that no deceptiveappearance of striation could possibly arise from irregular-ities in their surfaces.

The disc was first examined by inspection throughfaces cut upon its edges. These faces had not been re-polished, or brought to a true figure; so that nothingcould be gathered from this examination, but that nooffensive striae were seen through them.

The state of the disc, as to tension from imperfectannealing, was then tried by passing polarized lightthrough it at right angles and at obliquities, and analyzingthe emergent beam by tourmalines and Nichol prisms.It was found to be very remarkably free from indicationsof tension, only a feeble gleam of whitish light becomingperceptible on some portions of the edges, which mighthave arisen from the heat of \he hands of the personsemployed to lift and keep it in position.

Tt was then placed on edge, and tested by looking veryobliquely through it at objects offering broad lights andshadows. During this examination a group of stridebecame visible at a spot whose position on the glass wasmarked, and to which and its near neighbourhood theyseemed to be limited.

To trace the limits of tills group, and to subject thewhole disc to a more rigorous and delicate scrutiny, itwas set upright on a table; the room (the Indian tent)darkened, a candle placed 7 or 8 feet behind the disc, anda convex lens of about 5 feet focus and G inches diameter,of extremely pure glass, held between the candle and thelens close to the latter. The eye of the observer beingplaced before the disc in the place of the image of thecandle, so as to cover the lens with a glare of light; striae,it is well known, become distinctly apparent, if existing,in any part of the disc so covered.

Every part of the disc in succession, beginning fromthe centre, and proceeding outward in radii, was thuscarefully examined for striae. With the exception of ahair or thread, perfectly definite, and of a looped form,

thus, near the centre, and not of the smallest im-portance in an optical point of view, and a few othertrifling threads, originating apparently in minute sand-grains, in various points, and also utterly inoffensive, noappearance of striae whatever could be discerned over anypart of the disc, but at and in the immediate neighbour-hood of the place marked.

No bubbles of any consequence were noticed in this exa-mination, or on ocular inspection.

Attention being concentrated on the group of striae thuspointed out, it was found to occupy a space (at its utmostextent) of GJ inches in length and 2j inches in breadth,Beginning at about lj inches from the edge, and havingits longer dimension directed towards the centre. The

whole of this space was by no means equally affected,and the most objectionable portion was much morelimited. This was situated not far from the middle ofthe space in question; and it was proved decisively thatthe seat of the worst portion (or nucleus) of the groupwas almost close to one of the surfaces, hv the test ofparallax, viz., by placing a mark on the surface next theeye on the apparent place of the nucleus, and then tiltingthe disc right and left. One side (A) being towards theeye, the mark and nucleus remained coincident; and onreversing the disc,and bringing the other sided?; nearestthe eye, and making a fresh mark (effacing the former),they coincided only when the visual ray passed perpen-dicularly through the disc, the nucleus passing parallac-tically behind the mark, from side to side, on incliningthe disc.

As the disc would have to be worked into a concave orconvexo-concave lens, by which its thickness in thecentre would be reduced at least an inch, there is littledoubt, therefore, that by acting on that surface nearwhich the nucleus is situated, it, and much of the neigh-bouring stria?, which are evidently connected with it,would be ground out. Such is the decided opinion ofMr. Boss. In that case, as the whole area occupied bythe striated region does not exceed 0*019, or less thanone-fiftieth of the total surface of the glass, it might veryreasonably be expected that, if worked into an object-glass, the mass of good light would so far overpower theinjurious effect of what strise might remain, as to give avery satisfactory result, and that for many purposes ofhigh astronomy it might prove extremely efficient.

On the other hand, if it should be resolved to sacrificethe defective portion, or so much of it as to leave but a verytrifling part of a comparatively inoffensive region stillencroaching on the edge (suppose a spot of an inchdiameter), it would be practicable to cut out from thewhole disc a smaller one of 22 inches (29 — — 5j)

diameter, apparently quite perfect in every other part,and of 2*2 inches in thickness. But as this thickness issuperfluous, it might be found practicable, by cautiouslyheating the glass to the softening point, and applyingpressure, to extend it in surface whilst reducing itsthickness to 22-29ths of its present amount. Supposingthis operation successfully performed, and that in themanipulation some portion of the good glass adjacent tothe striated part being retained could be pressed inwards,so as to drive out the small striated corner, a disc of25 inches diameter, of apparently perfect flint glass, wouldbe produced.

The great object-glasses of Pnlkowa, and of New Cam-bridge in the United States , do not exceed 1G inches.Compared to this, a 25-incli, or even a 22-inch objectglass would be as great a stride in advance, as thePulkowa lens is beyond the largest achromatics previouslyconstructed.

On these grounds the Committee have had no hesita-tion in recommending that a Council Medal be given toMessrs. Chance for this disc. They desire it to beclearly understood, however, that no examination towhich a disc of glass unworked can be subjected, willafford more than a reasonable probability that a telescopeworked from it shall turn out perfect: since, independ-ently of peculiarities in the glass which this kind of exa-mination may fail in detecting (though no better has yetbeen devised), the perfection of the resulting instrumentwill still be dependent on that of the crown lens and ofthe optical workmanship.

Maes (France , No. G5G, p. 1209) has exhibited speci-mens of a new kind of glass; the basis of which is theoxide of zinc, a certain quantity of borax, a boracic acidbeing added, to give the glossy character for which theboracic compounds, no less than the silica, are so eminentlyremarkable, combined with an easier fusibility. As amaterial in the arts, the Jury have no further concernwith this glass, than as regards its probable utility in theconstruction of telescopes, prisms, and other opticalapparatus, for which it extreme limpidity and total free-dom from colour, and, so far as appears, from veins andstrice, seem eminently to fit it, and which have inducedthem to propose its being rewarded with a Medal. The