OBJECTIVES AND 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 

 dear, 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 visual 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 



