60 
ACCESSOEY APPARATUS. 
smaller bull’s-eye is used 
to bring the light to 
a focus, its flat surface 
should be turned towards 
the object. 
By following these 
directions, very little 
spherical aberration is 
produced by the form of 
the bull’s-eye, and the 
illumination is rendered 
more efiective than by 
any other position of the 
lenses. 
81- Smaller Biill’s-Eye 
Condenser. This instru- 
ment is mounted as shown in 
Fig. 29. It is very conven- 
ient for illuminating opaque 
objects to be viewed with 
low powers. The method of 
using it will be understood 
from the preceding section. 
J. & W. GRUNOW & CO’S ILLUSTRATED 
