THE TYPES OF OCULAR LENS. 25 



negative ocular the field-lens acts with the objective in 

 making clearer the image, and with the eye-lens in help- 

 ing to correct some of the aberrations. The eye-lens also 

 magnifies the image. 



THE TYPES OF OCULAR LENS. 



1. Compensating oculars, which correct the chromatic aber- 

 ration of the ray outside of the axis. 



2. Projecting oculars, used with the projecting microscope 

 or for microphotography. 



3. Spectroscopic oculars. 



These three types, among many, are the most important 

 and most frequently employed. 



The designation of oculars is by their magnifying power 

 and equivalent focal distance, and also by numbers, the smaller 

 number designating the lower power, and vice versa. 



The field of the microscope is the lighted portion which is 

 seen when one looks through the microscope with the instru- 

 ment in focus. 



The eye-point is the distance from the instrument at which 

 the eye may look through with the least strain. 



The Care of the Microscope. 



Keep the instrument cleaned, and see that all mechanical 

 parts move smoothly and evenly. Keep the mirror, con- 

 denser, and diaphragm central that is, in the optical axis. 

 Bring the object into view with the coarse adjustment, and 

 define the details in it by means of the fine adjustment. See 

 that no dirt or dust of any kind covers the lenses. Should 

 the field be blurred or dim, after proper focusing and light- 

 ing, the fault is either with the lenses or the cover-glass is 

 soiled. 



Tests for the Sources of Dimness in the Object. By revolv- 

 ing the ocular with the eye in position, the dimness, when 

 due to the ocular, will also move. By moving gently the 

 object with the hands, the dimness will move if due to dirt 



