THE MICROSCOPE AND MICROSCOPIC METHODS 29 



planes of the preparation.^ A proper appreciation of this micro- 

 scopical principle will at once suggest the importance of differential 

 staining methods in microscopy. 



Pig. 18. — Microscope 



The Bacteriological Microscope. — The bacteriological micro- 

 scope consists of a tubular body which carries the optical parts, and 



^ This principle may be crudely but clearly illustrated by covering a colored 

 glass bead with a deep layer of colorless glass beads in a test tube, so as to conceal 

 the former when looked at from above. By replacing the air between the beads 

 with homogeneous cedar oil the layer of colorless beads is rendered perfectly trans- 

 parent and the colored bead is distinctly seen from above. Filter paper may be 

 rendered transparent in a similar way. 



