402 Olservnl'ions on the Ocular Micromefcr. 



.steeple of Villc-Jiiif. The writer then states, that I manifested 

 a desire to procure one of those new instruments, which woulci 

 prouKite mv views exactly, and that he recommended to me Mons. 

 Soleil of Feydaii Passage, who at that time he says was making a 

 similar instrument for Dr. Gilbert «if Leipzig, as a proper man to 

 iriake me the instrument in question ; that this artist undertook 

 the conimir.sion at his request, and finished it by his direction, 

 l)cfore I left Paris. The author then takes some pains to prove 

 tiiat his instrument had been made and n^ed some years, and 

 savs that in accumulating proofs he apprehends he may be in- 

 juring vie, and therefore he suggests the propriety of my reply- 

 ing, as I presume, through the medium of your Magazine. In 

 a postscript Mr. Arago tells his readers, that a young Polish as- 

 tronomer. Dr. Slawinski, had arrived in Paris from London, and 

 had brouglit with him an instrument, made by an Englisli artist, 

 exacllij like those which Fortin had constrr.cted for the Obser- 

 vatory of Paris, except that the prisms were ten times thicker 

 than are made in Paris, and tliat consequently they would be zise- 

 les-; with a high power ; and lastly, he asserts that the exterior 

 faces of the pristn ought not to stand at right angles to the op- 

 tical axis of tiie telescope, and that he shall himself shortly pub- 

 lish a description of the Ocular Microrncier, together with all 

 necessary details respecting its construction, and the various uses 

 it has been put to in determining small angular measures. 



In this plausible narrative there is, Mr. Editor, such a strange 

 mixture of truth, conjecture, and misrepresentation, that I nnist 

 l)eg permission of vou to insert a quotation from the original 

 Memoir, in which I am supposed to have robbed Mr. Arago of 

 the originality of his invention ; and then to offer such remarks 

 on matters of fact, as will enable your readers to judge of the 

 propriety of the attack on my character which I have just no- 

 ticed. 



The Memoir begins with an explanation of Abbe Kochon'3 

 application of an achromatic prism of double refraction to mi- 

 crometrical measurements, states the defects of this mode of ap- 

 plying it, and then proceeds in these words ; viz. 



" On examining (at Lenoir's house) an object with the face 

 of the prism in the focal point, to satisfy myself that the image 

 then formed is a si'igle well-defined image, I had occasion to 

 adjust nicely for distinct vision; and in doing this I discovered 

 that, when the prism was out of the focal point, a pair of images 

 would be formed at the anterior side of this point, as well as at 

 the posterior, as they have reference to the eye looking through 

 the telescope ; and it appeared probable, that equal dibtances 

 on each side of the focal point gave equal measures, as far as the 

 small space left ue^r the eye-end of the tube would admit the 



prism 



