OBJECTIVES AND OCULARS. 
63 
marked with numbers showing their actual magnifying 
power. Thus, a Zeiss “ compensating ” ocular marked 
“8” magnifies the objective image eight times, and 
this conveys a definite meaning to us. 
There is some difference of opinion as to the use of 
the ordinary Huyghenian eyepiece for photo-micro¬ 
graphy ; but there is no doubt, first, as to the advan¬ 
tage of an ocular for this purpose when the result is a 
good image ; and second, as to the excellence of the 
projection oculars of Zeiss. The use of a suitable 
ocular lessens the length necessary for the camera, and 
prevents much of the danger of stray light reaching the 
plate. Good photo-micrographs have been produced 
with an ordinary ocular, but we have not succeeded 
with it, and we cannot but suspect that the successes 
of others have been fortuitous. Certainly, the ordinary 
eyepiece is not intended for this purpose; but, on the 
other hand, many fine results have been achieved with 
Zeiss’ compensating oculars, which were not intended 
for this work any more than the Huyghens oculars. 
At any rate, the projection oculars are strongly recom¬ 
mended for photo-micrography, and the compensating 
oculars may be found handy if, at any time, the pro¬ 
jection does not happen to give the desired magni¬ 
fication. 
It was found impossible to remove certain defects 
from the high power apochromatics without help of 
oculars, while these defects could be easily removed 
from the low powers; consequently, to obviate the need 
for a great many oculars, Zeiss leaves in his lower 
