MANIPULATION OF THE MICROSCOPE. 29 



The low-power objectives (those of one-half inch and 

 one-third inch equivalent focus) should always be used 

 for preliminary exploration before a new specimen is 

 examined with the higher powers, since their large field 

 of view makes the general relations of the object clear. 

 Focusing is also much easier with the lower powers, since 

 with them a small object may easily be found, while 

 their working distance, or the space between the objec- 

 tive and the object when in focus, is great enough to 

 allow free play. After placing the object over the center 

 of the opening of the stage, the tube should be run down 

 by means of the coarse adjustment until the front lens is 

 within an eighth of an inch of the object. Then, looking 

 through the microscope, the tube is slowly raised by the 

 coarse adjustment until the object comes more or less 

 clearly into view. It is often helpful to move the slide 

 about with the left hand at the same time, as the shadowy 

 outline of a moving object is more readily recognized than 

 that of one at rest. 



As soon as the object is dimly seen it may be brought 

 clearly into focus by the use of the fine adjustment. The 

 proper manipulation of this mechanism is of great im- 

 portance, since if the attempt be made to study objects 

 which are not well in focus, the eyes will be strained and 

 structures incorrectly seen. Furthermore, at any given 

 position of the fine adjustment only a certain plane of the 

 object examined will be in focus, while its whole figure is 

 needed to make up a correct picture. Therefore, one 

 hand should be kept continually on the fine adjustment 

 while the microscope is in use, varying it slightly as needs 



