45 



Description of Parts. As it is necessary for 

 the student to become conversant with the terms 

 of the various parts and to understand their use, 

 we give an illustration (Fig. 20) with letters 

 and append a list giving the names and recom- 

 mend that they be impressed upon the memory, 

 as they are the basis of microscopical language. 



A. Base. This is the foundation of the instru- 

 ment. It usually rests upon three points (or 

 should do so) and is of such a weight that it keeps 

 the instrument firm when it is in an upright or 

 inclined position. The two principal forms are 

 the tripod and horseshoe. The revolving plate, 

 when this is provided, by means of which the 

 upper portion of the instrument is revolved, with- 

 out changing the position of the base, is considered 

 a part of it. 



B. Pillar. 1 It is the vertical column which is 

 fastened to the base and carries upon its upper 

 end the joint or axis which is provided for inclin- 

 ing the instrument. It generally consists of one 

 piece either round or square but is often, in larger 

 instruments, made in two columns. 



C. Arm. This supports all the upper working 

 parts of the instrument and carries the provisions 

 for adjusting for focus. 



D. Body. This is the tube portion to which 

 the optical parts are attached. 



