276 USE OF THE MICROSCOPE 



short distance, and therefore the coarse adjustment should always 

 be used excepting for the finishing touches in getting a sharp 

 image. If at any time the fine adjustment has been run to the 

 extreme end of its range one way or the other the body tube 

 should be run up with the coarse adjustment and the micrometer 

 head of the fine adjustment should then be turned in the 

 requisite direction to bring the fine adjustment to about the 

 middle of its range. 



In some microscopes the condenser is provided with a focus- 

 ing screw, by means of which the condenser can be raised or 

 lowered to give the best results. Where there is a fixed mount- 

 ing for the condenser the latter should be kept with its front lens 

 flush with the upper surface of the stage. It is important that 

 the lenses of the condenser should be kept as crystal clear as 

 those of the ocular and objectives. 



For nearly all work the mirror bar should be kept parallel 

 with the long axis of the microscope, so that the light will be 

 reflected along the axis of the entire system of lenses. 



The clips (Fig. 147) are intended to press gently upon the 

 glass slide so that it can be moved about over the stage steadily 

 with gentle pressure of the fingers, but this cannot be done well 

 unless the stage is kept clean. 



It has already been said that w r here there is a condenser the 

 plane mirror should be used. The reason for this is that the 

 condenser is made to bring parallel rays of light to a focus in 

 the plane of the object, and if the concave mirror is used con- 

 vergent rays would enter the condenser and be brought to a 

 focus within the condenser system and the object would not 

 be so well illuminated. 



Keep the fingers off from the surfaces of the lenses, for they 

 are sure to leave a film that will need to be washed off. Most 

 of the trouble which beginners experience in the use of the 

 microscope comes from dirty lenses due to contact with the 

 fingers or reagents, and to water or other fluids in which the 

 object is mounted running out from under the coverglass and 

 coming between the latter and the objective. These troubles 



