CH. II] 



LIGHTING AND FOCUSING 



67 



the microscope. Place a drop of distilled water on the cover-glass, 

 and with the coarse adjustment ]ower the tube till the objective 

 dips into the water, then light the field well and turn the fine ad- 

 justment one way and another till the image is clear. Water im- 

 mersions are exceedingly convenient in studying the circulation of 

 the blood, and for many other purposes where aqueous liquids are 



Objective 



W Object-b 



Object-a 



Fig. 65. Figure to show that in lengthening the tube\of the microscope the 

 object 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 

 Jigure showing the effect of the cover-glass {Fig. 64) 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 y and if 

 the cover is so thin that it does not elevate the object enough for the corrections 

 ofj.he objective, then an increase in the tube-length will correct the defect. 



