Ch. II] 



LIGHTING WITH THE MICROSCOPE 



45 



amount of movement by | and the quotient will represent the total 

 thickness over the object. 



For example, in one case the microscope was focused on the ink mark 

 at the top of the cover, and then it was necessary to focus down i| 

 revolutions of the fine adjustment screw to bring the object in focus. 

 That is, it was necessary to focus down 0.15 mm. Now as this repre- 

 sents but § of the actual thickness of the cover-glass and mount- 

 ing medium over the object, the entire thickness was o.i5-=-§ = 

 0.2 2$ mm. Probably in this case the cover-glass was 0.15 mm. thick 

 and the object was in the mounting medium 0.075 mm - below the 



cover. 



Lighting with Daylight 



§ 83. Unmodified sunlight should not be employed except in special 

 cases (§ 125). 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 shown in fig. 33 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 ad- 

 vantage for one who is to use the micro- 

 scope for serious work that he should com- 

 prehend and appreciate thoroughly the 

 various methods of illumination, and the 

 special appearances due to different kinds 

 of illumination. 



§ 84. 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 microscope. By this kind of light objects are ordinarily seen by 

 the unaided eye and the simple microscope (fig. 4-5). In Histology, 



30 cm 



Fig. 33. Screen for 

 Shading the Microscope 

 and the Observer. 



It is composed of heavy 

 paper hung over a bent 

 wire, which in turn is an- 

 chored in a small tin dish 

 filled with lead. 



