Sir, 



82 



XV. — Note on Nachet' s Prism. By M. Nachet. 

 (Read April 17, 1850). 



Paris, April 10, 1850. 



I have received the note you sent me through your agent, and 

 have to thank you for the trouble you have taken on the subject of 

 the memoir read before the Microscopical Society. It is not at all 

 surprising that the author (whose name I could not well make out) 

 should have found that in our workmanship we were not theoretically 

 correct. After all, we have not failed through ignorance. 



We likewise have formulae which we apply to the construction of 

 my prisms. The whole question resolves itself into ascertaining the 

 best form of prisms for causing the emergence of the utmost 

 quantity of light through the terminal convex surface. In all these 

 discussions a large concession must always be made to the practical 

 part of the question, for the working of these prisms is sufficiently 

 ticklish to prevent the production of any two whose side-lengths are 

 exactly alike ; the angles, however, are always exact, but vary in an 

 ascertained manner, according to the different effects we may wish 

 to obtain. I would, however, remark that there are divers combina- 

 tions of angles, founded on the different inclinations that may be 

 given to the luminous rays in the interior of the prism ; for example, 

 in the prism of forty-five degrees (which does not merit the few 

 lines you have devoted in parentheses to acknowledgments) the 

 angles of reflection are obtained in a different manner from the 

 others. There are a great number of possible combinations, and I 

 am very desirous of knowing how the author has been able to arrive 

 at a general formula for all cases, and I would therefore beg you to 

 send me his memoir, at least in abstract. 



Yours, &c, 



Nachet. 



Mr. Warren De la Rue. 



