OBJECTIVES A-ND OCULARS 
3 
objectives are manufactured. When in the correction for chro¬ 
matic aberration three spectral color rays are brought to a common 
focus the objectives are known as apochromatic objectives. In 
these objectives the chemical and optical foci are identical and 
we have the highest grade of lenses at present available. Al¬ 
though in apochromatic objectives rays of three colors are brought 
to a correct focus, the images produced by these three sets of 
rays are not coincident and thus yield a colored fringe or halo 
at the edges of the field. This, however, is eliminated by em¬ 
ploying slightly over-corrected eyepieces, known as compensating 
eyepieces, in which the construction is such as to neutralize, or 
compensate for, the errors due to the objectives. Beautifully 
clear, colorless images are thus obtained, but the field is rarely flat. 
If the objectives are to be employed for the preparation of 
photomicrographs as well as for ^dsual observations, it follows 
that choosing between achromatic or apochromatic objectives 
becomes a rather puzzling question; for if ordinary achromatic 
objectives of high magnifying power are used the negatives may 
be lacking in fine details, while on the other hand if apochro- 
matics are employed the photographic images obtained are often 
so blurred at their edges as to be valueless as records save in 
the region about the center of the photograph. There appears 
to be a growing tendency toward the selection of achromatic 
objectives for metallographic microscopes and instruments 
intended for allied investigations where flat fields are highly 
desirable. The proper focus to produce clear sharp photographs 
is determined experimentally with each objective and a record 
kept in the notebook for future reference. 
Objectives are termed dry or immersion according as they are 
designed to be used with air or with some liquid between the 
front or lower lens and the preparation. High-power dry objec¬ 
tives must each be specially adjusted for a certain definite thick¬ 
ness of cover glass. In order to permit some freedom of choice 
in cover glasses many high-grade high-power dry objectives are 
adjustable and are provided with a movable graduated collar, 
permitting the adjustment of the objective for the thickness of 
the cover-glass used; that is, a part of the combination of lenses 
