PHENOMENA OF PHASE MICROSCOPES 27 



Suppose that, on the other hand, the deviated D wave of Fig. II. 4 is 

 retarded by an additional amount of 3^ wavelength with respect to the 

 undeviated *S wave by placing the refracting coating upon the com- 

 plementary area of the diffraction plate. The total retardation of the 

 deviated wave with respect to the undeviated wave is then 3^ wave- 



FiG. II. 7. Destructive interference between the undeviated S wave and the devi- 

 ated 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. The deviated wave 

 has been rendered H wavelength out of phase with respect to the S wave by the 

 introduction of an additional quarter-wave phase retardation into the D wave 

 beyond its nominal quarter-wave phase retardation with respect to the undeviated 

 S wave. The resultant wave is now weaker than the S wave. The particle will 

 therefore appear darker than its surround. Destructive interference is not complete, 

 because the amplitude of the undeviated wave still exceeds the amplitude of the 



deviated wave. 



length. Conseciuently, the undeviated and deviated waves interfere 

 destructively over the geometrical image of the particle, as indicated 

 in Fig. II. 7, to produce a resultant wave of smaller amplitude than that 

 of the undeviated wave. The particle therefore appears darker than its 

 surround. In summary of the last two paragraphs, a particle whose 

 optical path exceeds that of the surround by a small amount will appear 

 brighter or darker than its surround according as the optical path of the 

 conjugate area of the diffraction plate is increased or decreased by 34 



