ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
261 
(4) Photomicrography. 
New Method of Illumination 
for Photomicrographical Pur- 
poses.* — Dr. A. Kohler’s method 
of illumination consists in so ad- 
justing the relative positions of 
the condenser beneath the object 
and the source of light that an 
image of the source is formed 
approximately in the plane which 
is the common base of all of the 
pencils proceeding from the pro- 0 
jection system to the several 
points of the image. This plane 
will usually be near the hinder 
focal plane of the objective. The 
source of light must therefore 
be placed approximately in the 
hinder focal plane of the con- 
denser. The course of the rays 
in this case is seen in the upper 
part (above the line A B) of 
fig. 23. Lj L 2 is the source of 
light in the hinder focal plane 
of the condenser C. O is the 
plane of the object in which three 
points Oj o o 2 are marked. The 
position of the object is chosen 
so that it is beyond the focal 
length of the condenser, p is 
the objective serving for the pro- 
jection, with the iris diaphragm J 
in its hinder focal plane. In this 
plane there is produced an in- 
verted real image l x Z 2 of the 
source of light L x L 2 . 
The angular aperture of the 
pencil which meets the plane of 
the object in the optic axis is 
equal to the angle under which 
the virtual magnified image of 
the source of light X 2 , produced 
by the condenser, appears from 
the plane of the object. This 
can be regulated by diaphragms 
placed directly in front of the 
source of light. The figure shows 
that the illumination of the whole 
field of view is uniform, even if 
* Zeitschr. f. wiss. Mikr., x. (1893) 
pp. 433 -40. 
