34 



LIGHTING AND FOCUSING. 



[CH. II. 



high objectives, it would be necessary to use very thin cover-glasses over the ob- 

 ject. (See \ 22, 27). If too thick covers are used, it may be impossible to get an 

 objective near enough an object to get it in focus. For objects that admit of ex- 

 amination with high powers it is always better to use thin covers. 



LIGHTING WITH DAYLIGHT. 



\ 58. Unmodified sunlight should not be employed except in special cases. 

 North light is best and most uniform. When the sky is covered with white clouds 

 the light is most favorable. To avoid the shadows produced by the hands in 

 manipulating the mirror, etc., it is better to face the light ; but to protect the eyes 

 and to shade the stage of the microscope some kind of screen should be used. 

 The one figured in (Fig. 58) is cheap and efficient. If one dislikes to face the 

 window or lamp it is better to sit so that the light will come from the left as in 

 reading. 



It is of the greatest importance and advantage for one who is to use the micro- 

 scope for serious work that he should comprehend and appreciate thoroughly the 

 various methods of illumination, and the special appearances due to different 

 kinds of illumination. 



Depending on whether the light illuminating an object traverses the object or is 

 reflected upon it, and also whether the object is symmetrically lighted, or lighted 

 more on one side than the other, light used in microscopy is designated as re- 

 flected and transuiilfed, axial and oblique. 



39- 40. 



Figs. 39-40. For full explanation see Figs. 22 and 23. 



\ 59. Reflected, Incident or Direct Light.— By this is meant light reflected upon 

 the object in some way and then irregularly reflected from the object to the micro- 

 scope. By this kind of light objects are ordinarily seen by the unaided eye, and 



