OPTICAL FUNDAMENTALS 7 



the sine curve D, which is almost exactly 3^ wavelength out of phase 

 with the curve S, provided that the retardation of P is small. D 

 represents the light deviated by diffraction at the specimen. The light 

 represented by S acts as if no specimen were present; i.e., it is undevi- 

 ated. *S and D represent the two waves entering the microscope 

 objective. If we were to obstruct S by blocking it off in the objective 

 lens system, we would have simply darkfield illumination; if we were 

 to obstruct D and let *S through, we would have no detailed imagery 



Fig. 1.2. Deviated ray retarded by 34 wavelength. 



of the specimen. Microscopic images in general are obtained by allow- 

 ing both the undeviated wave, S, and the deviated (by diffraction) 

 wave, D (Fig. 1.2), to pass through the microscope objective and inter- 

 fere in the neighborhood of the geometrical image of the particle formed 

 by the objective. Here th e waves D a nd 8 recombine and fo rm the 

 w ave P. The image of the surround is formed by the wave S. Since 

 the amplitudes of S and P are equal, because the specimen is transparent, 

 there is no contrast between the image of the particle and its background, 

 and the particle is invisible. This situation is characteristic of the ordi- 

 nary microscope. 



Now, if we can change the phase of D with respect to S so that *S and 

 D are either in exactly the same phase or 3^ wavelength out of phase, we 

 can obtain striking contrast between the specimen and its background. 

 That this shift in phase can be accomplished by artificial means will 

 be shown presently. Figure 1.3 represents the combination of the waves 

 D and ^S when the phase of D has been advanced (or that of *S retarded) 



