28 



MICROSCOPE AND ACCESSORIES 



CH. /] 



ing the objective in forming the real image, and an eye-lens which magnifies 

 the real image. While 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 image achromatic. 



Oculir lo 2 



FIG. 37. Compensating Oculars of Zeiss, with section removed to show the 

 construction. The line A- A is at the level of the upper end of the tube of the 

 microscope while B-B represents the lower focal points. It will be seen that 

 the mounting is so arranged that the lower focal points in all are in the same 

 plane and therefore the microscope remains in focus upon changing oculars. 

 ( The oculars are par-focal.) The lower oculars 2, 4 and 6 are negative, and 

 the higher ones, 8, 12, 18, are positive. The numbers .?, 4, 6, 8, 12, /, indicate 

 the magnification of the ocular. From Zeiss' Catalog. ) 



\ 46. Compensating Oculars. These are oculars specially constructed 

 for use with the apochromatic objectives. They compensate for aberrations 

 outside the axis which could not be so readily eliminated in the objective it- 

 self. An ocular of this kind, mangifying but twice, is made for use with high 

 powers, for the sake of the large field in finding objects; it is called a search- 

 ing ocular; those ordinarily used for observation are in contradistinction 

 called working octilars. Part of the compensating oculars are positive and 

 part negative. (Fig. 37.) 



47. Projection Oculars. These are oculars especially designed for pro- 

 jecting 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 ach- 

 romatic objectives of large numerical aperture. The projection oculars 

 (Fig. 38) consist of a collective lens or field lens and of a carefully corrected 

 system for the eye lens The eye lens is movable so that a sharp image of 

 the diaphragm between the field and eye lens may be projected upon the 

 screen at different screen distances. 



\ 48. Micrometer Ocular. This is an ocular connected with an ocular 



