PHENOMENA OF ALL MICROSCOPES 



19 



is incident upon the object specimen, we note that one portion of the 

 incident wave is intercepted by the surround and that the remainder 

 of the incident wave is intercepted by the particle. We denote the 

 portion of the incident wave that is intercepted by the surround as the 

 S wave and the portion that is intercepted by the particle as the P wave. 

 Because we have supposed that neither the particle nor the surround 



Fig. II. 4. Graphical relations among the P, S, and D waves such that D + S = P. 



The P wave is shown as retarded with respect to the & wave by a small phase angle, 



A, in radians. The constructed D wave is retarded with respect to the S wave by 



approximately X/4 wavelength or 7r/2 radians. 



absorb light, the amplitudes of the P and S waves should be equal to 

 each other and to the amplitude of the incident wave. Suppose that 

 the optical path difference A between the particle and surround is a 

 small fraction of a wavelength and that the optical path of the particle 

 exceeds that of the surround. The P wave will then lag behind the 

 *S wave by the amount A, as in Fig. II. 4. Now, it is possible to construct 

 after the manner indicated in Fig. II. 4 a wave T> that, when added to 

 the *S wa\'e, produces a resultant wave identical to the P wave. In 

 other words, it is possible to find a D wave such that D -{- S = P. The 

 only graphical requirement is that the ordinates y of the D and S waves 

 shall add up to equal the ordinates y of the P wave. The pair of waves 



