PHENOMENA OF PHASE MICROSCOPES 29 



undeviated wave has been reduced to the ampHtude of the deviated 

 wave by passage through an absorbing material placed on the conjugate 

 area of the diffraction plate. The undeviated and deviated waves now 

 coincide and produce by constructive interference a resultant wave 

 whose amplitude is twice as large as the amplitude of the undeviated 

 wave. Because brightness is proportional to the square of the amplitude 

 and because the image of the particle is illuminated l)y the resultant wave 

 whereas the image of the surround is illuminated by the undeviated 



.Undeviated and weakened Swave 



■^Deviated and artificially retarded D wave 



Fig. II. 8. Complete destructive interference between the undeviated S wave and 

 the deviated D wave as they overlap the sharply focused image of the particle after 

 passing through a diffraction plate that equalizes the amplitudes of the undeviated 

 and deviated waves and that renders these two waves different in phase by 3^ w^ave- 

 length. The particle will appear black against a background of reduced brightness. 



wave, the image of the particle is foin* times as bright as the image of 

 the surround. The effect of adding the absorbing material to the 

 conjugate area is, therefore, to reduce the brightness of the surround 

 more rapidly than the brightness of the particle until the particle 

 appears distinctly brighter than the surround. In principle, the relative 

 brightness of the particle can be increased in this manner until the 

 conjugate area is opaque and the surround is black as in the well-known 

 Schlieren method. 



Although the particle appears with maximum bright contrast in the 



