26 AN ELEMENTARY THEORY OF PHASE MICROSCOPY 



the undeviated wave will be retarded by \^ wavelength. This selective 

 retardation follows directly from the fact that the undeviated wave 

 passes through the conjugate area whereas the deviated wave passes 

 through the complementary area. The effect of retarding the undevi- 

 ated *S wave by Y^ wavelength is to slide the entire »S wave of Fig. II.4 



Fig. II. 6. Constructive interference between the undeviated S> wave and the 

 deviated D wave as they overlap the image of an object particle whose optical path 

 difference A with respect to its surround is small and positive. In the phase micro- 

 scope, the undeviated and deviated waves have been made to agree in phase by- 

 introducing a phase retardation of \i wavelength into the undeviated wave with 

 respect to the deviated wave, whose relative phase retardation is nominally }/i 

 wavelength with respect to the S wave. The resultant wave is stronger than the 

 undeviated S wave. Consequently the particle will appear brighter than its sur- 

 round in the plane of sharpest focus. 



to the left by }/i wavelength. The relation between the deviated and 

 undeviated waves becomes that illustrated in Fig. II. 6. It can be seen 

 that the undeviated and deviated waves interfere constructively to 

 produce the indicated resultant wave of greater amplitude than that 

 of the ;S wave. Since the geometrical image of the particle is illuminated 

 by this resultant wave whereas the image of the surround is illuminated 

 by the <S wave alone, the particle appears brighter than its surround. 



