USE OF THE MICROSCOPE 13 



the tube slowly downward until the objective nearly 

 touches the slide, watching it carefully from the side, and 

 then raise it by the fine adjustment until the focus is es- 

 tablished. Never focus down with the high-power objec- 

 tive because of the danger of pressing it into the slide and 

 ruining the delicately mounted lenses. 

 D. Studying an object : 



1. Always examine an object first with the low powers so 

 as to understand its general structure before passing to 

 details. 



2. Obtain greater magnification, if the instrument permits, 

 by using more powerful objectives rather than higher eye- 

 pieces, for owing to peculiarities of the lenses clearer 

 images are thus obtained. 



3. Do not rest satisfied until the light is of the best quality 

 obtainable with the mirror and diaphragm. It should not be 

 too bright. Details are shown more clearly by subdued light. 



4. Keep both eyes open in using a microscope. If this is 

 at first distracting, cover the free eye with the fingers 

 or by a paper screen projecting from the microscope tube 

 until it is no longer attracted by surrounding objects 

 on the table and the attention is entirely concentrated 

 on the working eye. Never let the habit of squinting 

 develop. 



5. If it is necessary to ascertain the exact size of an object 

 this can best be done by the use of two micrometers. The 

 eyepiece micrometer consists of a disk of glass ruled with 

 fine equidistant lines ; this is inserted beneath the upper 

 lens of the eyepiece. The stage micrometer is a glass slide 

 ruled with fine lines 1-100 mm. apart. To measure an 

 object the number of spaces on the eyepiece micrometer 

 which its image covers must be noted. Then the value of 

 each space is to be ascertained by substituting for the 

 object the stage micrometer. A simple calculation will 

 now give the diameter of the object. 



