26 BACTERIOLOGY 



more distinctly visible will the object be. Uncolored trans- 

 parent objects are visible by virtue of their ability to refract light 

 and so to present darker and lighter zones. If the surrounding 

 medium possess the same refractive power as the colorless trans- 

 parent object, the latter is invisible. 1 Microscopic objects may 

 conceivably be invisible or so nearly invisible as to have escaped 



FIG. 15. Showing the manner in which the "dark outline picture" is produced. 



(After A. E. Wright.) 



detection for this very reason. If, however, the object be sus- 

 pended in a medium of lower refractive index, then it may be de- 

 nned by light and shade, and it is most clearly defined when illu- 

 minated in one of two ways, either by a rather narrow direct beam 

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

 case the object is denned by dark outlines upon a white field; or by 



1 This may be illustrated fairly well by immersing clean, perfectly clear glass 

 beads in oil of cedar wood. 



