CH. II] 



LIGHTING AND FOCUSING 



57 



mam* other purposes where aqueous liquids are liable to get on the 

 cover-glass. If the objective is adjustable, follow the directions given 

 in § 103. 



Objeetive 



Object-b 

 Object-a 



Fig. 58. Figure to show that in lengthening the tube of the microscope the ob- 

 ject must be brought nearer the principal focus or center of the lens. It will be seen 

 by consulting the figure that in shortening the tube of the microscope the object 

 must be removed farther from the center of the lens. By consulting the figure 

 showing the effect of the cover-glass {Fig. 57) it will be seen that the effect of the 

 cover-glass is to bring the object nearer the objective, and the thicker the cover the 

 nearer is the object brought to the objective. As shortening the tube serves to remove 

 the object, it neutralizes the effect of the thick cover, and if the cover is so thin that 

 it does not elevate the object enough for the corrections of the objective, then an in- 

 crease in the tube-length will correct the defect. 



