MICROMETERS 



95 



dimly lighted. Raise the tube on the coarse adjustment very slowly 

 and gradually. As soon as the faintest details can be seen, perhaps 

 the margin of the drop or a few individual bacteria, use the fine 

 adjustment to get the exact focus. 



In examining both stained and unstained bacterial preparations 

 it is desirable to move the slide in order to examine the whole speci- 

 men; this necessitates constant changing of the fine adjustment in 

 order to keep the object in focus. 



FIG. 37 



Attachable mechanical stage. 



Micrometers. In the study of bacteria their size is always men- 

 tioned. The task of measuring so small an object as a bacterium 

 must appear a very formidable affair to the student, yet it is an 

 exceedingly simple procedure. It, however, requires the use of 

 some microscopic accessories, such as a stage micrometer and an 

 eyepiece micrometer. The stage micrometer consists of a glass slide 

 with a finely ruled scale of one millimeter, divided into 100 equal 

 parts, hence the space between two dividing lines is equal to one- 

 hundredth of a millimeter, or 10/z = 10 micra = 10 micromillimeters. 

 The simplest eyepiece micrometer consists of a circular glass disk 

 with a finely ruled scale which divides a line in the centre of the disk 

 into fifty equal portions. This disk should be laid upon the inner 

 diaphragm of the eyepiece. In order to do this the upper lens of 

 the eyepiece must be unscrewed and then replaced again. Some 

 eyepiece micrometers are very complicated and expensive, but all 

 are made on the same principle, a line in the centre divided into 



