62 
PRACTICAL PHOTO-MICROGRAPHY. 
slightly thicker than usual. Objectives up to the eighth 
of an inch are generally made “ dry ” ; eighths are 
sometimes dry and sometimes immersion. We have a 
dry 3 mm. glass of Zeiss (apo. J), and also a lens of 
equal power, apo. oil immersion N.A. 1.4; each lens is 
excellent for its own purposes. 
Objectives of higher power than half-inch are usually 
made with “ correction collars ” ; these collars are for 
the purpose of altering the relative position of the com¬ 
binations of the lens, to meet various thicknesses of 
cover-glasses. In practice the collar is of great import¬ 
ance towards securing the best definition, and careful 
study and practice should be devoted to the use of the 
collar. But in the case of the apochromatic immersion 
glasses of Zeiss, the correction for cover-thickness is 
achieved by alteration of the length of the optical tube, 
as has already been mentioned. In selecting objec¬ 
tives it is necessary to be careful as to the power 
quoted by the maker, not only for the reason given 
above (p. 58), but on account of the fact that one 
objective will stand the use of a more powerful ocular 
than another objective may stand. Thus, when we 
compare a one-sixth with a quarter-inch, we may find 
that the quarter will give a good image with a high 
power ocular, while the sixth will break down under the 
same ocular. In fact, there is no better rapid test of 
an objective than to use it with a strong ocular. 
Oculars in this country are marked by arbitrary letters, 
A, B, C, etc., A being the lowest power; the system of 
Abbe or Zeiss is far preferable, for their oculars are 
