6 PRACTICAL HISTOLOGY [l 



objective into position, and remember to do this always 

 when shifting from the low to the high power. In 

 shifting from one objective to the other, see that the 

 one in use is properly centred ; focus a specimen with 

 one objective, then move it a little way from the stop 

 and note the difference in the appearance and illumi- 

 nation of the specimen. 



7. When the definition of a section is not good, 

 examine the slide and cover-slip, the upper and lower 

 lenses of the ocular and the front surface of the objec- 

 tive to see that all of these are clean. Specks of dirt 

 on the lenses of the ocular will of course move on 

 turning round the ocular; and specks of dirt on the 

 specimen on moving the slide. 



A piece of soft chamois leather and a piece of silk 

 should be kept in a small dust-tight box, for the pur- 

 pose of cleaning the lenses ; if this is not at hand, th 

 cleaning should be left to the attendant. 



Dust on the lens should be removed by lightly 

 flicking it with silk. 



Glycerine or any aqueous solution should be re- 

 moved by streaming the surface with water from a 

 wash-bottle, and then dabbing it lightly with silk or 

 chamois leather to dry it. 



Canada balsam, turpentine, clove oil, grease should 

 be removed by placing a drop of xylol or benzine on the 

 surface of the lens; dabbing this with silk, and repeating 

 the process several times. 



8. Thickness of cover-slip. Use of higher objectives. Objectives 

 are generally corrected for a cover-glass of medium thickness ; 

 (0'15 to 0-2 mm.), and cover-slips thicker than 0'2 mm. should 



