Observalivns on the Ocular Micromtier. 40i> 



prism to go: from this circumstaiiL-e I i:iimediatcly coiicliideil 

 that two images niiglit also probaljiv be formed hv a pribm, after 

 the rays Iiad proceeded through the lenses constituting the eye- 

 piece, but did not ])orcei\ e, at l!;e time, what the measure of an 

 angle would be that might probably be so obtained. 1 desired 

 Mons. Lenoir to adapt me a double pristn of clear crystal to a 

 cell, that I might aj)ply it to llie eye-end of a telescope, j'or the 

 {)urpose of making e?<|)erimeiits on this mode of application ; on 

 wiiich he laughed at tiie idea, and assured me that as the angle 

 of every prism is co/islanf, there is no otlier way but ti)at adopted 

 by Rochon, by which a vcrielij of measures could be taken ; 

 when I endeavcmred to explain to him how Dr. Brewster had 

 varied t/ie meusure,\n his patent telescope, by a variation in the 

 power, a term which it appeared he did not understand, till I 

 hit upon the word awpUjication. At length Mons. Lenoir un- 

 dertook to comply with my wish of his fitting a prism into a cell ; 

 but he had no itlea that a small prism, somewhat larger than 

 the pupil of the «Lye, would have been sufficient for my purpose, 

 and therefore he mounted one large enough to be used on Ro- 

 chon's principle. 



"While tliis prism was preparing, I visited Mr. Arago at the 

 Royal Observatory ; and on informing him what I had in hand, 

 he appeared surprised, and fetched Irom a ))rivate cup-board or 

 drawer, a celestial eye- piece, with a small prism oclualh/ applied 

 to it in a cell, in the way I had ordered Lenoir to fit up his. I 

 then learnt that the objection, as to indistinctness of vibion, 

 which applies to Hochon's construction, is olu-iated by this new 

 application cf his prism, by means of which the image, regularly 

 formed without previous transmission of the rays through the 

 crystal, is viewed double; and I have since found, that the eye- 

 lenses only modify the measure of the angle of the two images, 

 as seen through them and the prism conjointly. What Mr. 

 Arago had delermiricd this modification to be, he did not inlorm 

 me, except that he tabulated, from experiment, the angular 

 measures which residted from diiferont arrangements of his lenses, 

 that produced different amplifications ; and that the angles so 

 measured are very small ; but the ol)jects appeared, he said, 

 much more distinct tlian when the images are formed by Ro- 

 chon's Micromctrical Telescope. 



"On my return from the Royal Observatory, where I learnt that 

 Soleil made the small ])risms, I applied to this optician to n>ake 

 me about half a do/en, similar to those which he had made for 

 Mr. Arago, to be adapted to a similar eye-piece; Imt 1 was in- 

 formed, that he knew nulliing of the construction of Mr. .Arago's 

 eve- piece, nor to what ptnpoic he applied liis jirisins." 



7'lie .MtiTioir then relates how I determined what hmscs 

 3 E 2 would 



