5 i6 GENERAL BIOLOGY 



Breathe upon one side of the cover to moisten it, and let 

 down with one edge in advance of the other. 



6. The air may cling to the object. For exampl 

 mount a dry thread in a drop of water, cover and examir 

 with low power. Alcohol may be used to remove the ai 

 Remount the thread in alcohol ; cover, allow a little time 1 

 soak, and observe the disappearance of the air. Air bul 

 bles are generally present in freshly mounted slides, an 

 must be recognized, and not confused with structure 

 Note the peculiarities of their refraction at different foci, an 

 learn to recognize an air bubble instantly. 



On locating dirt that is in the field of vision. 



1. It may be on eyepiece, objective, slide, condenser ( 

 mirror. To tell whether it is on the eyepiece, rotate the ey( 

 piece while looking through it ; if there, the dirt will rotal 

 with the eyepiece. 



2. To tell whether it be on the objective, change focu 

 if there, it will give the same obscurity at all foci. 



3. If it be on the slide, it may be located by moving tt 

 slide or cover; it will move with the slide, and may readil 

 be focused upon; the two sides each of slide and cove 

 offer four levels at which it may be found. There is n 

 excuse for using dirty slides or covers, or for allowing dii 

 upon condenser or mirror, where it may be directly observe 

 and whence it is easily removed. 



On ike use of mirror and diaphragm. 

 i. Learn the use of the mirror : i) by trying both sid< 

 of it; 2) by trying it in different positions of the swingin 

 mirror bar; and 3) by taking light from various sources, 

 the wall, the ceiling, the curtain, the sky. Then keep th 

 mirror bar vertical, and use the flat side with, the conca\ ; 

 side without, a condenser. 



