FUNDAMENTAL CONSIDERATIONS 13 



3. Find the light by looking into the ocular (eye piece) and at the 

 same time turning the mirror at such an angle that it reflects light 

 from the window or lamp up through the opening in the stage to 

 the objective. When opaque objects are to be illuminated, a 

 stronger illumination is required than that usually afforded by an 

 ordinary laboratory lamp or by the light from a window. For this 

 purpose a microscope lamp, such as the Spencer no. 374 (see Fig. 

 5) is very satisfactory. Mirrors have two faces, a plane and a con- 

 cave. Use the concave unless employing the condenser, when the 

 plane mirror should always be used. 



4. Regulate the quantity of light by the diaphragm. If too bright 

 it must be cut off somewhat. The higher powers require brighter 

 light than the lower. 



5. Place the slide on the stage in a horizontal position with the 

 object over the middle of the opening through which light is thrown 

 from the mirror. 



6. With the lower power in position, move the coarse adjustment 

 until either the object or small solid particles on the slide appear 

 distinctly, which means that the lenses are in focus. The object, 

 if not under the lens, may now be brought into the field by moving 

 the slide back and forth very slowly while looking through the ocular. 

 To improve the focus, slowly turn the fine adjustment screw. 



7. To focus with- the high-power objective, first find the object 

 with the low power and arrange in the center of the -field. Put clips 

 on slide without moving it. Raise the body tube by means of the 

 coarse adjustment. Then turn the high-power objective into posi- 

 tion. (If two objectives only accompany your instrument, the high- 

 power is the longer one.) Lower the body tube carrying the objec- 

 tive until the objective front lens nearly touches the cover glass. 

 A slight movement of the fine adjustment should show the object 

 clearly. Never focus down with the high-power objective while 

 looking through the ocular because of the danger of pressing it into 

 the cover glass and ruining the delicately mounted lenses. 



8. Accustom yourself to use both eyes indifferently and always 

 keep both eyes open. If right handed, observe with the left eye, 

 as it is more convenient in making drawings. 



9. When the oil immersion objective is to be used, a small drop of 



