242 MANUAL OF ANIMAL BIOLOGY 



(c) The mirror, with concave and plane surfaces, which is at- 



tached underneath the stage. 



(d) The rody, with coarse and fine adjustments for focussing, 



draw ture, eyepiece, or ocular, and revolving nose- 

 piece holding the orjectives. 

 Learn how each of these parts is used. 



3. Place the microscope in position, with the upright support toward 

 you. See that the low-power eyepiece No. 1 and the low-power objective 

 No. 1 are in position and that the iris diaphragm is open. Adjust the 

 mirror so that the maximum amount of light will be reflected up through 

 the central opening of the stage. In general, use the mirror with the 

 concave surface up. 



4. Place the glass slide, on which the object to be examined is mounted, 

 upon the stage with the object directly over the central opening. Care- 

 fully turn the coarse adjustment screw away from you until the end of the 

 objective is approximately one-fourth of an inch from the upper surface of 

 the slide. Then, while looking through the eyepiece, slowly turn the coarse 

 adjustment screw toward you until the object comes into view. Bring 

 the image into an exact focus by slowly turning the fine adjustment 

 screw in either direction, as may be necessary. Never focus down- 

 ward with the coarse adjustment while looking through the micro- 

 scope. It is less tiring and better in every way to keep both eyes open 

 while looking through the microscope. You will be able to do this with 

 a little practice. 



5. Always find an object first with the lowest magnification, as described 

 in the preceding paragraph, and always use the lowest magnification 

 possible to accomplish the work in hand. Before changing from a lower 

 to a higher power, always see that the object is exactly in the center of the 

 field. The magnification may then be increased as desired in the following 

 ways: 



(a) By replacing low-power eyepiece No. 1 with high-power eye- 

 piece No. 1 



(ft) By replacing low-power objective No. 1 with high-power ob- 

 jective No. 1 This is accomplished by revolving the nose- 

 piece. Inasmuch as this objective, when in focus, is very 

 close to the upper surface of the preparation, it will be 

 necessary to use great care in making this change. 



(c) By doing both (a) and (ft). 



(rf) The magnification can be still further increased in some 

 microscopes by extending the draw tube, but this method 

 is rarely used. 



1 The numbers are different in the various makes of microscopes. The correct 

 numbers for your microscope will be supplied by the instructor. 



