6 4 
PRACTICAL PHOTO-MICROGRAPHY. 
powers these defects, which are corrected by his com¬ 
pensating oculars ; and it will be found that the apo- 
chromatics do not perform well for photo-micrography 
without either a projection or a compensating ocular. 
But most of the achromatic lenses of modern make 
answer very well without eyepiece; we are in the 
frequent habit of using several glasses in this way. 
Two fine very low power glasses by Zeiss are made for 
use without ocular—the 70 and 35 mm.—and Zeiss’ aa 
is also most valuable for use alone. In this way we 
have used also low power glasses by Swift, Beck, 
Crouch, and Watson. Zeiss makes projection oculars 
numbered “ 2 ” and “ 4 ” for the short tube stands, and 
3 and “ 6 for the long tube. These oculars work 
well with many lenses not apochromatic that have been 
in our hands. 
We may call the magnification given by an objective 
at the distance of ten inches from its posterior conju¬ 
gate focus the “initial power” of that objective; and 
for guidance it may be stated that a good objective will 
stand the test of photography at about six times the 
initial power ; a very good glass will stand ten times ; 
nothing but a very fine glass indeed will tolerate being 
strained more than this, and it does not matter how 
the tax of magnification is brought about, whether by 
stretch of camera, by heavy eyepiecing, or by these 
combined. Thus, the initial power of a Zeiss apo. 
3 mm. glass is 83; as soon as we try to make a negative 
at about 800 diameters, we begin to find difficulties, 
which only a master of the science can overcome. It 
