24 GENERAL BIOLOGY 



known as focusing. In the last of these operations will be 

 involved learning the uses of the so-called coarse and fine 

 adjustments, the former effected by means of the rack motion 

 produced by the large milled heads at the side of the arm, 

 and the second by means of the small milled head at the top 

 or side. Try out all these operations in a general way and 

 finally under the directions of the next section. 



3. Focusing. The object to be examined must be thin, 

 since the light must pass through it. The object is placed 

 on a glass slide, with a drop of water or other liquid, and 

 covered with a thin glass cover. Place the slide on the stage 

 with the object directly over the center of the opening. 

 Move the low power objective rather close to the slide and, 

 while looking into the eyepiece, turn the coarse adjustment 

 so as to raise the tube until the object comes into view, 

 now using the fine adjustment focus carefully until the image 

 is perfectly sharp. 



The high power, w y hen in focus, is so close to the cover 

 glass that great care must be used in adjusting it or it may 

 be injured. Turn the nose piece slowly to bring this ob- 

 jective into position, taking care to see that it does not 

 touch the cover glass. If it swings into position without 

 striking, a slight movement of the fine adjustment will usually 

 bring the image into sharp focus. If the high pow r er can- 

 not be swung into position the same process of focusing 

 must be used as for the low power. Always focus upward, 

 since to do the contrary may result in the lens striking the 

 slide to the injury of both the slide and the lens. 



Practice these points patiently and with care until every 

 phase is clearly understood and easily managed. This 

 practice in manipulation will be time w T ell spent, since later 

 work will thereby be done easily and skilfully. 



