reflections from the mirror of trees, window sash, etc , 

 they will show on the back lens of the objective when 

 it is focused and when the condenser is in this posi- 

 tion. The operator must decide for himself as to 

 whether he gets better results with the sharp focus and 

 the images, or without either. If he decides to retain 

 the images he can get best results by turning the mir- 

 ror so that they are as symmetrically distributed over 

 the back lens as possible. This emphasizes the impor- 

 tance of a clear, open source of light. 



CENTERING THE CONDENSER 



For central light the axis of the condenser should 

 coincide with the axis of the objective and the center 

 of the opening of the diaphragm beneath the conden- 

 ser should also be in this axis. On most of the medium 

 grade microscopes the iris is so fastened to the conden- 

 ser that it is concentric with its axis, and both are gen- 

 erally centered with the optical axis of the body tube 

 before leaving the factory. This can be tested by a 

 simple means. Close the diaphragm to its smallest ap- 

 erture and notice this aperture through the back lens 

 of the objective. If the condenser is centered this 

 opening will appear in the center of the lens and will 

 remain concentric with the periphery of the lens when 

 the objective is focused up and down. The periphery 

 of the top surface of the condenser will also be con- 

 centric with the periphery of the back lense of the ob- 

 jective. 



On the more expensive microscopes there are little 

 screws provided for moving the condenser in and out of 

 center. By means of these the condenser can be cen- 

 tered by observing the above rule. On these instru- 

 ments the iris diaphragm is brought in and out of center 

 by a rack and pinion, a click indicating when it is cen- 

 tered. 



22 



