26 BACTERIOLOGY 



differs from that coining from its surroundings either in quantity 

 or in quality, and the greater the extent of this difference the 

 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- 



PiG. 15. — Showing tlie manner in which the "dark outline picture" is produced. 



{After A, E. Wright.) 



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

 conceivably be invisible or so nearly invisible as to have escaped 

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

 suspended in a medium of lower refractive index, then it may be 

 defined by light and shade, and it is most clearly defined when 

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



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

 beads in oil of cedar wood. 



