ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
561 
(fig. 86) enclosed between two flint-glasses b b\ of which b has a higher, 
and b' a lower refractive index than a. Practically, however, the first 
combination, flint between crowns, is found to be most suitable. 
Fig. 85. Fig. 80. 
Fig. 87. 
As an example, fig. 87 shows in natural size an objective of this kind, 
of 240 mm. focal length, and 30 mm. greatest effective aperture. 
For the production of this objective the necessary constants are as 
follows : — 
Eadii of Curvature. Glass -Thickness. 
Rj = — Kg = 45-835 
R 2 = — R 7 = 54-324 
R 3 = — E 0 = 19-853 
R, = - R 5 = 49-088 
d,= d G = 7-343 
d 2 = d l = 1-833 
d 3 = d i = 4*584 
A = 11-00 air-distance. 
Kinds of Glass. 
jiD. nG. 
L, = L 6 : 1-61310 1-62683 
Lo = L 5 : 1-56804 1-58182 
Isl = L 4 : 1-51497 1-52663 
