77 



The few prisms of Nachet's own manufacture which I have had 

 an opportunity of examining do not appear to be constructed exactly 

 according to the formulae I have given above ; from what cause I am 

 at a loss to conjecture, as I have no doubt of the correctness of these 

 formulse. I abstain from entering into the mathematical details, 

 simply from a feeling that they would interest few, if any, of the 

 members of the Microscopical Society. 



Fig. 2. 





i v\ *• -. 



