ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
137 
Similarly, for the flint lens wo have f } = 5/3 with 
Anteplane = 5/16 and postplane = 0. 
In the figure the principal planes of the flint lens arc represented 
by f 1 f 2 , those of the crown lens by c x c 2 , and those for the whole doublet 
by d, d v 
The transit distance for the two lenses / 2 — c L = 2/5. 
Hence the first principal focal length of the doublet is 
, ff£ = 30 
fc — [ Ij -*r t 13 
The same processes are applied to the other doublets A and B, 
which are then combined into a low power objective, whose principal 
planes have the positions marked A B x and A B 2 . This is then combined 
with the front doublet to form the total objective, whose principal planes 
are marked 0 2 in the figure. Similarly, the equivalent lens for the 
eye-piece is determined, and finally the eye-piece is combined with the 
objective. The transit distance in the last case depends on the tube- 
length, and was 220 ’8432 for the Microscope used by the author. The 
total focal length was found to be ‘623, while the lower principal piano 
was 1‘18 below the front vertex V 0 at the point M x in the figure, and 
the upper one at 6 ’794 above the eye-piece. The anterior focal length 
is positive, and its focus is represented by F. 
The Effect of Curvature of the Cover-glass upon Micrometry.*— 
Dr. M. D. Ewell writes “ Inasmuch as by far the greater number 
* Proc. Amer. Soc. Micr., xii. (1891) pp. 79-83. 
