OBJECTIVES AND OCULARS. 
57 
by colour-corrected plates ; and for the most critical 
and perfect work we still require objectives of the 
apochromatic type. 
Objectives have been produced called “ semi-apo- 
chromatic ” by some Continental opticians, notably by 
Leitz and Reichert, and we can corroborate the high 
opinion of these lenses given in Dr. Dallinger’s edition 
of “Carpenter.” The editor of that book speaks highly 
of several objectives of this type, and he says of one— 
Reichert No. 6—that it is the “ rival of even true 
apochromatics.” 
Briefly, we recommend those who aim high, and who 
can afford them, to use apochromatic objectives; failing 
these, semi-apochromatics; but good work can be done, 
sufficiently good for many purposes, if not for most, 
with well-chosen achromatics of the most modern con¬ 
struction. The vast majority of apochromatics are 
made by Zeiss, but Powell and Lealand make apo¬ 
chromatics not inferior, and often superior, to those of 
Zeiss at a somewhat lower price, as we know from 
practical experience. 
Definition—and consequently “resolution”—depends 
on “ numerical aperture.” Definition is the highest 
quality an objective can possess, and so we ought to 
look for high aperture in our lenses. But the crux of 
practical optics is to get high aperture with low power. 
“ Power ” depends on the focal length of the lens ; the 
shorter this length the higher the magnifying power. 
It is easy-to get high angle with high power, but the 
