MICROSCOPICAL EXAMINATION. 911 
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-gilass. 
