CHAPTER IV. 
OBJECTIVES AND OCULARS. 
\ S a matter of course, the greatest care should be 
taken to provide one’s self with the best optical 
appliances that can be procured; but the difficulty 
in choice which existed a few years ago is now practi¬ 
cally overcome by the advance in optics that has been 
made. It is now more a question of expense than any¬ 
thing else. In past times, objectives were made excellent 
in every respect for ordinary ocular observation, but 
not “ corrected for photography.” 
It is well known that in consequence of the unequal 
refrangibility of those parts of the spectrum which 
chiefly exercise chemical activity, and of those parts 
which chiefly act upon the retina, the foci of the 
chemical and visual rays passed through a lens do not 
correspond in position ; that, while an image might 
appear accurately in focus to the eye, it might not im¬ 
press itself in accurate focus on a photographic plate. 
During years now passed, objectives were made and 
called “ achromatic,” but seldom was attention given 
to eliminate this defect, which did not interfere with 
the performance of the objectives for visual observa¬ 
tion ; but when attention began to be paid to photo¬ 
micrography, opticians were forced to make objectives 
