6 STUDENTS HISTOLOGY 



(with the low -power this will be less than one -half of an inch), 

 and hereafter you will be able to focus more quickly. 



Having observed the details of structure as shown with the low- 

 power, swing the high -power into use. Rack the tube down until 

 the objective is very near to the glass covering the object. The 

 field is much obscured. Watching the effect through the eye- 

 piece, rack the tube up with great care until the image appears 

 sharp. Note the distance with this objective, as before with the 

 low -power, from one -twelfth of an inch to considerably less. 

 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 improve- 

 ment in definition. The rule is: The higher the power, the smaller 

 the diaphragm. 



You have doubtless observed, before this, that you cannot con- 

 trol the focusing 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 focusing -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 focusing under the 

 most perfect control. 



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 and 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 

 manipulation with the fine adjustment will be required. It will 



