6 THE FROG 
Place an object, mounted on a slide and covered with a cover 
glass, in the center of the stage. Adjust the concave mirror and 
the diaphragm so that light passes through the object. 
B. Use of the Low Power.—With the low power, objective in 
line with the tube, and the eye on a level with the stage, turn the 
coarse adjusting screw so as to lower the tube until the objective 
is nearer the object than the stated focal distance (usually 3 inch). 
With one eye at the ocular looking down through the tube, turn 
the coarse adjusting screw slowly so as to raise the objective until 
the object comes clearly into view, With the coarse adjustment 
focus carefully. 
Move the slide about so as to see all parts of the object. Ob- 
serve the apparent reversal of direction of the motion, and the in- 
version of the image. Are right and left also reversed? Open 
and close the diaphragm slowly, observing the effect. The best 
results are secured with an aperture barely large enough to admit 
sufficient light. 
In working with the low power it is usually best to use only 
the coarse adjustment for focusing; but when only slight changes 
of focus are needed these may be secured with the fine adjust- 
ment. Test the action of the fine adjustment and observe that 
it works slowly and within a very limited range; the screw must 
not be turned more than one or two complete rotations in the 
same direction. 
C. Use of the High Power.—Before using the high power 
always see that the object is in perfect focus and im the exact 
center of the field of view with the low power. Then turn the 
high power objective into place. If the microscope is in perfect 
adjustment (the two lenses parfocal), the object will now be in 
focus with the high power. But if, as is often the case, the 
lenses are not strictly parfocal, place the eye at the level of the 
stage and very carefully lower the tube by means of the coarse 
adjustment until the objective almost touches the cover glass of 
the object slide—that is, nearer than the focal distance of the 
high power objective (usually % inch). With the eye at the 
ocular then raise the tube slowly until the object comes clearly 
into view; finish the adjustment with the fine screw. Readjust 
mirror and diaphragm until best results are obtained. While 
