IO THE FROG 



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 ad- 

 justing screw slowly so as lo 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. 

 Observe the apparent reversal of direction of the motion, 

 and the inversion of the image. Are right and left also re- 

 versed? 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 adjustment. Test the action of the fine adjust- 

 ment 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. 



B. Use of the High Power. Before using the high 

 power always see that the object is in perfect focus and in 

 the exact center of the field of view with the low power. 

 Then turn the high power objective into place. If the micro- 

 scope 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 gla^s of the object slide that is, 

 nearer than the focal distance of the high power objective 

 (usually y^ inch). With the eye at the ocular then raise 



