THE MICROSCOPE AND MICROSCOPIC METHODS 



25 



scope is tte substage illuminating apparatus, consisting of the 

 mirror, the iris diaphragm and the condenser. These are neces- 

 sary to illuminate minute objects so that they may be satis- 

 factorily studied at high magnifications. By the use of the iris 

 diaphragm and of the central spot stop, the ordinary condenser 

 may be made to furnish three different kinds of illumination, (i) 

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

 hollow come of light converging on the object at a wide angle, an 

 example 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, 11 and 12. 

 Dark-field illumination is obtained in 



Fig. 13. — Dark-field condenser showing 

 optical parts and centering mechanism. 



Pig. 14. — Optical parts of 

 the dark-field condenser with 

 object slide and microscope ob- 

 jective with funnel stop in possi- 

 tion. The path of light rays 

 is indicated by the dotted lines. 



a more satisfactory manner by employing a special condenser 

 ma,de for the purpose, illustrated in Figs. 13 and 14. The way 

 in which these different methods of illumination affect the visi- 

 bility of a colorless refractive object is illustrated in Figs. 15,16 

 and 17. 



Visibility of Microscopic Objects. — In the use of the micro- 

 scope 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 



