ABSORPTION GRATINGS 276 



for equally spaced, pure phase gratings illuminated by narrow cones of 

 axial illumination. 



10.2. Absorption gratings viewed under narrow-coned axial 

 illumination 



The main thesis of this subsection is to demonstrate theoretically that 

 phase microscopy is useful not only for improving contrast in the image 

 of pure phase gratings (object specimens whose adjacent portions differ 

 only in optical path) but also for improving contrast in the image of pure 

 absorption gratings (object specimens whose adjacent portions differ 

 only in amplitude transmission). This theoretical prediction has led 

 to the use of the phase microscope in observing weakly stained biological 



u 



g i_ 



— I 1 I — 



f(Xo) 



g g V 



U 1 ,11 ' ^^ 



I I 



I 1 



h< 21 ^, 



Fig. VII. 10. Simply periodic, pure absorption gratings as object specimen. The 

 grating spacing is 21 along the A'o direction. The troughs and elevations are taken 



as equal in width. 



specimens. The method of this section and of Section 10.1 can be 

 extended to show also that the phase microscope is useful for improving 

 contrast in the image of object specimens ^^'hose adjacent portions 

 exhibit differences in both optical path and amplitude transmission. 

 The utility of the phase microscope in observing pure phase or pure 

 absorption specimens has been established in Chapter II on the ele- 

 mentary theory but can, of course, be established more conclusively 

 with the aid of the more general theory. 



It is pointed out that with the use of narrow-coned, axial illumination 

 all th-e higher spectral orders will pass through the complementary area 

 except with object gratings that have unusually large spacings 21. For 

 most practical purposes the zero order will therefore be altered imiformly 

 with respect to the higher orders when the specimen is observed under 

 narrow-coned, axial illumination. 



In the simple, equally spaced grating of Fig. VII. 10 let the amplitude 



