6 PRACTICAL MICROSCOPY. 



the object. The field is much obscured. Watching the effect 

 through the eye-piece, rack the tube down with great care until the 

 image appears sharp. Note the distance with this objective as before 

 with the low power, probably about one thirty-second of an inch. 

 Then endeavor, by slight alterations in the inclination of the mirror, 

 to increase the illumination. Turn the diaphragm so that the light 

 passes through a small opening, and note the improvement in defini- 

 tion. The rule is : The higher the poiver, the smaller the diaphragm. 



You have doubtless observed, before this, that you cannot control 

 the focussing as easily as when the low power was in use. Slight 

 movements of the rack-work produce marked changes in definition ; 

 and it is difficult, with the coarse adjustment alone, to make as 

 slight movements as you may desire. Recourse must be had to the 

 fine adjustment. 



Place the tip of the forefinger (either) upon the milled-head of the 

 fine focussing-screw, and the ball of the thumb against its side, so 

 that the hand is in an easy position. By a little lateral pressure the 

 milled-head may be turned slightly either way. Note the effect on 

 the image. You thus have the focussing under the most perfect con- 

 trol. 



Remember that the fine adjustment is only necessary with high 

 powers, and then only after the image has been found with the coarse 

 adjustment. 



METHOD IN OBSERVATION. 



The study of objects under the microscope should be conducted 

 with order or method. 



The body being in the position before advised, so that the sitting 

 may be prolonged without fatigue, let one hand be occupied in the 

 maintenance of the focal adjustment. It will be found, however 

 flat an object may seem to the unaided eye, that as it is moved so as 

 to present different areas for examination (and with the higher 

 powers only a small area can be seen at once) constant manipula- 

 tion with the fine adjustment will be required. It will also be found 

 that even the various parts of a simple histological element like a 

 cell cannot be seen sharply with a single focal adjustment. The 

 forefinger and thumb of one hand must be kept constantly on the 

 milled-head of the fine focussing screw. Supposing the light to be 

 on our right, we devote the right hand to the focussing. 



The left hand will be engaged with the glass slip upon which the 

 object has been mounted. The forearm resting upon the table, let 

 the thumb and forefinger rest on the left upper side of the stage, just 



