THE MICROSCOPE AND MICROSCOPIC METHODS 25 



central illumination by a narrow beam, (2) illumination by a hollow 

 cone of light converging on the object at a wide angle, an ex- 

 ample of dark-field illumination, and (3) intense illumination by a 

 broad beam converging at a wide angle upon the object. These 

 possibilities are illustrated in Figs. 10, n and 12. Dark-field 

 illumination is obtained in a more satisfactory manner by em- 

 ploying a special condenser made for the purpose, illustrated in 

 Figs. 13 and 14. The way in which these different methods of 



FIG. 13. Dark-field condenser showing optical FIG. 14. Optical parts of 



parts and centering mechanism. (After Leitz.} the dark-field condenser with 



object slide .and microscope 

 objective with funnel stop in 

 position. The path of light 

 rays is indicated by the dotted 

 lines. (After Leitz.) 



illumination affect the visibility of a colorless refractive object 

 is illustrated in Figs. 15, 1 6 and 17. 



Visibility of Microscopic Objects. In the use of the mi- 

 croscope it is necessary to pay some attention to the factors upon 

 which visibility depends. An object may be distinguished and 

 perceived by the eye only when the light coming from the object 

 differs from that coming from its surroundings either in quantity 

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



