28 BACTERIOLOGY 



beam of light passing from behind it directly toward the eye, in 

 which case the object is defined by dark outlines upon a white field; 

 or by oblique beams directed at an angle from the sides, when the 

 object is defined by bright outlines on a dark background. If, 

 however, the object be illuminated from all sides or from behind and 

 from both sides by light of similar intensity, its outlines become 

 less distinct and may even be completely obliterated so that the 

 object becomes invisible. These facts may be crudely illustrated 

 by holding a test-tube full of water, (i) between the eye and a 

 window, (2) between the eye and a dark wall between two win- 

 dows, and (3) against the center of the window pane. Their 

 importance in microscopy may be readily illustrated by examining 

 a simple preparation of living bacteria, (i) with the iris diaphragm 

 nearly closed, (2) with the dark-field condenser, and (3) with the 

 ordinary condenser with the iris wide open. It will be evident 

 that the third arrangement is fatal to the definition of colorless 

 transparent microscopic objects. It will also be observed that 

 the dark field offers an advantage in the ease with which the ob- 

 jects can be seen, the small luminous outline on the dark back- 

 ground being more distinct than the dark outline on the luminous 

 background. The former might be compared in this respect 

 to a star at night, and the latter to a sun spot in the daytime, 

 which though many times larger may not be readily perceived. 

 The method of making objects visible by a difference in quality 

 of light (color) usually involves the necessity of staining. Colored 

 preparations have certain very important advantages for micro- 

 scopic study. If an object can be differentially colored, that is, 

 stained a different color or a different shade of the same color 

 from the material by which it is surrounded, it becomes clearly 

 visible even in the absence of different refractive power. Refrac- 

 tion may be largely eliminated by replacing the fluids of the 

 preparation by other fluids of high refractive index, such as 

 cedar oil or balsam, and this elimination of refraction eliminates 

 the opacity of the preparation, "clears" it, and makes possible the 

 distinct definition of minute objects situated in the deeper optical 



