44 



PROGRESS IN MICROSCOPY 



Their number is assumed to be very large. Spacings between the slits 

 are opaque and of the same width as the latter. This is, in fact, 



Fig. 1.54. Edge of area imaged in coherent and partially coherent illumination. 



a grating which is substituted for the specimen P and observed in 

 monochromatic light (Fig. 1.55). Naturally, in Fig. 1.55, the sHt width 

 is unduly large. As the direct light from the source S impinges against 



Fig. 1.55. Image of a periodic object in coherent illumination. 



1 1 1 



Fig. 1.56. Diffraction by parallel slits. 



the object P, each sht, such as AB or A^B^ diffracts some of the in- 

 cident light. Let us consider the phenomena taking place at slit AB 

 and AyBy (Fig. 1.56). The vibrations dilTracted by the slit media O 



