34 



AN ELEMENTARY THEORY OF PHASE AHCROSCOPY 



with respect to the optical path through the complementary area. Fur- 

 thermore, contrast in the image will be reversed as the refractive index 

 of the surround is increased from a value below that of the particle 

 to a value slightly higher than the refractive index of the particles. 

 Theorems 3 and 4 may, therefore, be applied to a variety of object 

 specimens. 



Diffraction plate 



^Condenser 

 diaphragm 



Fig. II.9. Passage of the spectral orders from an object grating through the phase 

 microscope. Only selected rays of the zero and first orders are shown, in order to 

 avoid confusion. In reality the grating spacing is much smaller than illustrated. 

 The rays of the zero and first order are drawn as full and broken lines, respectively. 

 The lines with short dashes illustrate that portion of the first order which has been 

 deviated upward at the object grating. The lines with long dashes illustrate that 

 portion of the first order which has been deviated downward at the ol)ject grating. 

 C is the image of C as formed by the light in the zero order. Cj^n is the image of C 

 as formed by the higher orders n = 1, 2, 3, etc. Note how the undeviated and 

 deviated rays from, for example, point a in the object grating are focused by the 



objective to form the image a' of a. 



3.5. Passage of the undeviated and deviated waves from a simply 

 periodic object grating through a phase microscope 



It is highly instructive to examine, if only briefly, the paths followed 

 by the undeviated and deviated waves that arise by diffraction at a 

 simple diffraction grating employed as the object specimen and which 

 then pass through the phase microscope. Light rays from a point C in 

 the condenser diaphragm emerge from the substage condenser as a 

 substantially parallel bundle of rays, as in Fig. II. 9. One portion of this 

 bundle of rays is undeviated by the grating and continues through the 



