ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 823 



Fi and Fj denoting the dioptric power of objective and eye-piece, d^ the 

 space between the first principal point of the objective and the second 

 principal point of the eye-piece, do the distance of this latter point from 

 the first nodal point of the eye, V the distance of accommodation, and q 

 the distance of the object from the first focus of the objective. 



More recently M. Monoyer has arrived at a formula more particularly- 

 applicable to spectacles, viz. 



^■=;-^' (-) 



A comparison of different formulfe for the magnification shows that 

 the majority of authors, such as Martin, Deschanel, Jamin, use formulre 

 belonging to the form 



in which V is the distance of distinct vision. 



The magnification thus defined answers to the comparative amplifying 

 power, if we make I = I', 



The consideration of the formulae proposed by Eees, Verdet, and 

 Guebhard shows that these authors, in order to appreciate the influence 

 of the lens on the visibility of an object, have had recourse either to the 

 magnification or to the relative amplifying power. M. Panum alone has 

 calculated the absolute power. The conclusion drawn by the author is 

 that M. Monoyer's formula j)ossesses a degree of generality and simplicity 

 which warrants its adoption in preference to all others. 



In the experimental determination of the amplifying power of the 

 Microscope, use is made of the fact shown by formula (6) that under 

 two circumstances the amplifying power F,- becomes equal to the 

 dioptric power F, viz. when L' = or when d^ = f. A determination 

 of the dioptric power for the latter case when the second focus coincides 

 with the first nodal point of the eye, consequently gives the amplifying 

 power. 



Two methods for determining the dioptric power are given, dis- 

 tinguished as the method of precision and the rapid method. 



The method of precision depends on the first formula of magnification 



Two measurements of the magnification are taken with the object 

 placed successively at two different arbitrary distances. The cor- 

 responding magnifications are 



/ 



Gi = 



G.= l 



whence 

 and 



Gi — G2 



F = , 



i'h - qi) Gi Gj 



