OPTICAL FUNDAMENTALS 9 



parallel rays. The specimen is an important part of the optical system 

 because of its action on the light-wave surface perpendicular to the" 

 rays. The specimen diffracts the light, provided that it is inhomoge- 

 neous with respect to the refractive index and/or absorption of light. 

 The deviated light is represented by the rays diverging from the specimen 

 and originating there. The parallel rays continue their course, without 

 deviation, into the microscope objective system and are brought to a focus 

 at its back focal plane, whereas the bundle of deviated rays is focused at 



Fig. 1.4. Decreased brightness from subtractive summation of direct and deviated 



light. 



the focal plane of the eyepiece by the objective lenses. At the diffraction 

 plate* the direct and de\'iated bundles are definitely segregated with only 

 very little overlapping. The phase of either the direct or the deviated 

 light can therefore be changed by introducing a layer of dielectric 

 material, such as magnesium fluoride, on that portion of the diffraction 

 plate covered by these rays. In Fig. 1.5 the direct light is retarded by 

 the layer of dielectric material introduced at the small central portion 

 of the diffraction plate to increase the optical path. For most systems 

 the phase retardation introduced here should be 3^ wavelength, as shown 

 in Figs. 1.3 and 1.4, in order to produce maximum contrast in the image. 



* The term "diffraction plate" was originated by the authors and is used in prefer- 

 ence to "phase plate," which, although emphasizing that this plate changes the phase 

 between the deviated and the direct light from the specimen, does not indicate that 

 it is of equal importance that this plate should absorb a part of the energy of one set 

 of rays. The designation "diffraction plate" refers to the entire action of this plate 

 on the diffraction spectra produced by the specimen. 



