28 



PROGRESS IN MICROSCOPY 



Before going into these phenomena as they occur in a microscope, 

 the two methods of illuminating a specimen will be set forth. In the 

 Kohler method, the source ^o is imaged by means of the lens L on 



Fig. 1.32. Kohler illumination. 



the iris diaphragm D^ of the condenser C ^o and L form the illumi- 

 nator of the microscope and may be either separate or integral with 

 the instrument. 



The specimen is at P; the objective O^ images it at P' where it is 

 observed through the eyepiece O2. Against the lens L is the iris 

 diaphragm Z)o- The position of the condenser (C and A) is adjusted 

 so as to image the diaphragm Z)o on the specimen P. Next, Dq is 

 diaphragmed to the extent that only the field actually observed is 

 illuminated. In this way, spurious light is reduced to a minimum 

 because even the specimen areas outside the visible field may transmit 

 light within the microscope. Some of this light always reaches the 

 image and the resulting halo weakens contrasts. 



Figure 1.33 shows the other method of illumination: the condenser 

 C images the source Sq into the specimen P. Figure 1.33 is simplified 



Fig. 1.33. Critical illumination. 



because, by means of an additional optical system and another 

 diaphragm, only the specimen field actually observed need be illumi- 

 nated, as in the Kohler method. But this method may result in uneven 

 illumination and show, superimposed on the object image, a spurious 

 image possibly exhibiting luminance variailons. 



However that may be, theory shows that both Kohler and critical 

 illumination take effect, from the coherence standpoint, in exactly 



