APPENDIX I 665 
convex crown lens and a concave flint lens. Objectives like 
oculars are usually designated by fractions as 149, 1é, 24, etc., or 
in millimeters, as 2 mm., 4 mm., 16 mm., which represent focal 
y 
EP Evepoiat 
o era | 
paiement 0; men] 
wok 
A 
ss i 
{| a! 
A 
y | 
i! \ 
c \ 
(Eo 
te,’ 
1 
: H Enteric: 
Pencil 
mans 2a 
Paral 
Light % 
; 3 3 
Fic. 493.—Diagram illustrating optics of a compound microscope in use. Fy, 
Upper focal plane of objective; F2, Lower focal plane of eyepiece; A, Optical tube 
length = distance between F, and F2; Ou, object; Oz, real image in F., transposed 
by the collective lens, to Os, real image in eyepiece diaphragm; Ou, virtual image 
formed at the projection distance C, 250 mm. from EP, eyepoint; CD, condenser 
diaphragm; L, mechanical tube length (160 mm.); 1, 2, 3, three pencils of parallel 
light coming from different points of a distant illuminant, for instance, a white 
cloud, which illuminate three different points of the object. (Courtesy of Bausch 
and Lomb Optical Co.) : 
lengths or numbers, as 97, 45, 10, etc. which indicate the mag- 
nification of the objective alone. - 
The smaller the number in millimeters or fraction represent- 
ing the focal length of an objective, the greater is its magnifying 
power. : 
