MICROSCOPICAL EXAMINATION. 211 



ferent materials under examination. It is necessary 

 to remember that they are seen with difficulty, and that 

 we are very apt, unless extremely careful in focussing, 

 to allow the lens to go too far, and so come upon the 

 cover-glass, break it, destroy our preparation, and, if 

 examining parasitic bacteria, infect the lens. This may 

 be avoided by first finding the hanging drop with a low 

 power lens and thus exactly centre it. The lens of 

 higher magnification is now very gradually lowered, 

 while at the same time gently moving the slide back 

 and forth to the slightest extent possible with the left 

 hand. If any resistance is felt raise the lens, for it has 

 gone beyond the point of focus and is touching the 

 cover-glass. 



