404 Observations on the Ocular Micrometer. 



would be suitable for an eye-piece having variable powers, atxi 

 that Soleil constructed it solely under mv directions; and, as I 

 l)elieve, without having any communication whatever with Mr. 

 Arago on the su'oject while I remained in Paris ; for he adapted 

 the eye-piece to an old telescope of his own, to satisfy himself that 

 its properties were really what I had ver!)allv explained to him ; 

 and he seemed so jdeased with the result, thai he offered his 

 services to execute any orders for nie, that 1 might afterwards 

 give him from England. 



From the preceding extract, vour readers will perceive, that 

 I havfi acknowledged Mr. Arago's prior claim to the application 

 of a prism of double refraction to the eye-piece of a telescope; 

 and that the circumstances that led me to think of the same 

 contrivance, antecedently to my visit to the Observatory, are 

 fairly stated ; and I may add, that I have vet by me the prism 

 and cell made by Lenoir, the order for which was given some 

 days before 1 saw Mr. Arago, or knew any thing about his eye- 

 piece. It will also be seen that the conversation about an ocular 

 micrometer commenced with me; and that Mr. Arago fetche(| 

 out his instrument after I had informed him of what / had in 

 hand ; and perhaps your readers will think this gentleman's 

 charge a little extraordinary, vvhen I tell them further, that he 

 took especial care that I should not examine the interior struc- 

 ture of his eye-piece, for he put it awav again almost imme-^ 

 diately after he had allowed me to see that it would give two^ 

 images, when a|)plied to a telescope; but how nianv lenses there 

 might be, or what might be their respective focal lengths, an^ 

 distances between them, were data I was left to conjecture! 

 neither was I apprised of any optical theorem on which the mea- 

 sures depend: and on that account these considerations consti- 

 tute the main subject of my second Memoir. Indeed I havei 

 some reason to conclude, that Mr. Arago derived the resulting 

 nieasures of his micrometer from experiments onlv; for 1 happen 

 to know, that he ])reviously obtained from a London optician a 

 very good dynamotcr, with which lie was highly delighted, that 

 would give him, with very little trouble, the powers of his tele- 

 scope in all the positions of his lenses, independently of theory. 



With respect to Mr. Arago's fancying that I went to the French 

 Observatory for t'.ic purpose of learning from him how to ma- 

 nage gt)od observations with French instruments, I do not (juarrel 

 with that; for, as I have long been in the possession of better in- 

 struments than any I saw in Paris, 1 could have no objection to 

 learning so important a secret. But how it happened that both 

 the Editors of the ylnnalcs de C/iimie forgot my address, 1 have 

 yet to learn ; and I can only account for it, by supposing that 

 they have both forgotten that I took them in my own carriage to 



the 



