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 

 clear, colorless images are thus obtained, but the field is rarely flat. 

 Objectives are either 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 ob- 

 jectives must each be specially adjusted for a certain definite 

 thickness of cover glass. In order to permit some freedom of 

 choice in cover glasses most high grade high power dry objectives 

 are adjustable and are provided with a movable graduated collar, 

 permitting the regulation of the objective for the thickness of the 

 cover glass used; that is, a part of the combination of lenses 

 making up the objective may be raised or lowered in the mount- 

 ing, thus affording a correction for the displacement of the image 

 brought about by the cover glass. By consulting the diagram, 

 Fig. 19, page 44, it will be seen that by turning the collar C the 

 combination of lenses L will be displaced and their distance 

 from the combination L' will either be increased or diminished. 

 A cover glass which is thicker than that for which the objective 

 is corrected affects the image in the same manner as if the spheri- 

 cal aberration were over-corrected, while on the other hand if 

 too thin the effect produced is similar to that of under-correc- 

 tion. In the first case the focal distance of the objective must 

 be increased, and in the second, decreased. This is accomplished 

 by turning the adjusting collar to the right or left, as the case 

 may require, or, in the absence of such a device, by shortening 

 or lengthening the distance between the eyepiece and the objec- 



