5i6 GENERAL BIOLOGY 



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

 down with one edge in advance of the other. 



6. The air may cUng to the object. For example, 

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

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

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

 soak, and observe the disappearance of the air. Air bub- 

 bles are generally present in freshly mounted slides, and 

 must be recognized, and not confused with structures. 

 Note the peculiarities of their refraction at different foci, and 

 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 or 

 mirror. To tell whether it is on the eyepiece, rotate the eye- 

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

 with the eyepiece. 



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

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



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

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

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

 offer four levels at which it may be found. There is no 

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

 upon condenser or mirror, where it may be directly observed 

 and whence it is easily removed. 



On the use of mirror and diaphragm. 

 I. Learn the use of the mirror: i) by trying both sides 

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

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

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

 mirror bar vertical, and use the flat side with, the concave 

 side without, a condenser. 



