40 



PROGRESS IN MICROSCOPY 



light diffracted by A which ends at A^^. This time, however, these 

 two illuminations, the coherent ground and the diffracted light, although 

 not independent from the vibratory standpoint since they are originated 

 by the same pin-point source Si, are coherent. Hence it is not both 

 ground and the diffracted hght-intensities that are to be added together 



Fig. 1.50. Extended object P in coherent illumination. 



but those of the amplitudes of the luminous vibrations reaching Aq, 

 taking into account the path difference or the phase difference of these 

 vibrations. When the condenser diaphragm D^ is opened, illumination 

 becomes partially coherent and the image structure is usually difficult 

 to calculate. 



Small black disk on white ground imaged in coherent and partially 

 coherent illumination 



Let us first consider a small black disk imaged in coherent illumi- 

 nation, i.e. when D^ is almost fully diaphragmed down. Results 

 are shown by curve 2 (Fig. 1.51). The structure is similar to that 

 shown in Fig. 1.38 but the fringes are much more prominent. 

 Figure 1.52 conveys much information: it shows how the image 



Fig. 1.5 1. Small black, disk on white ground imaged in coherent illumination. 



