CH. /.] 



MICROSCOPE AND ACCESSORIES. 



25 



the field-lens aids the objective in the formation of the real, inverted image, and 

 increases the field of view, it also combines with the eye-lens in rendering the im- 

 age achromatic. 



\ 36. Compensating Oculars. — These are oculars specially constructed for use 

 with the apoclirouiatic objectives. They compensate for aberrations outside the 

 axis which could not be so readi^' eliminated in the objective itself. An ocular 

 of this kind, magnifying but twice, is made for use with high powers, for the 

 sake of the large field in finding objects; it is called a searching ocular; those 

 ordinarily used for observation are in contradistinction called working oculars. 

 Part of the compensating oculars are positive and part negative (Fig. 31). 



J 37. Projection Oculars. — These are oculars especially designed for projecting 

 a microscopic image on the screen for class demonstrations, or for photographing 

 with the microscope. While they are specially adapted for use with apochromatic 

 objectives, they may also be used with ordinary achromatic objectives of large 

 numerical aperture. 



Fig. 32. Projection Oculars with section re- 

 moved to show the construction. Below are 

 shown the upper ends with graduated circle to in- 

 dicate the amount of rotation found necessary to 

 focus the diaphragm on the screen. No. 2, No. 

 4.. The numbers indicate the amount the ocular 

 magnifies the image formed by the objective as 

 with the compensation oculars. {Zeiss' Catalog, 

 No. jo}. 



\ 3S. Micrometer Ocular. — Tliis is an ocular 

 connected with an ocular micrometer. The 

 micrometer may be removable, or it may be per- 

 manently in connection with the ocular, and ar- 

 ranged with a spring and screw, by which it may 

 be moved back and forth across the field. (See 

 Ch. IV). 



13 Fig. 34. 



Figs. 33-34 Ocular Micrometer with movable scale. Fig. 33 is a side view of 

 the ocular while Fig. 3/ gives a sectional end view, and shows the ocular microme- 

 ter in position. In both the screw which moves the micrometer is shown at the 

 left. {From Bausch & Lomb Opt. Co.) 



