278 THE DIFFRACTION THEORY OF MICROSCOPY 



The even orders are absent with equally spaced object gratings. We 

 suppose that all orders higher than the third fail to reach the plane of 

 the image. 



In order to formulate a problem which can be solved with relative 

 ease, we suppose further that the condenser diaphragm is opaque except 



Conjugate area 



Edge of clear aperture 



-3' CD 

 I 

 I 



Fig. VII. 11. Arrangement of the spectral orders at the plane of the diffraction plate. 



The +3 and —3' branches of the third order are blocked. The first orders pass 



through the complementary area. The zero orders and and the branches +3 



and —3 of the third spectral orders pass through the conjugate area. 



for two small holes whose centers are arranged on a line perpendicular 

 to the lines of the grating. The grating is therefore illuminated by two 

 narrow cones of oblique illumination. The holes in the condenser 

 diaphragm shall be chosen so small that one is willing to accept the 

 approximation 



pfVo^K\^h_o^\^ (10.3.1) 



\M 2MI m) \M 2Ml) 



The optical direction cosine po is of opposite sign for the light issuing 

 from the two holes. If one looks down the body tube of the microscope 

 after removing the eyepiece, the arrangement of the spectral orders 

 will have the appearance of Fig. VII. 11. The actual conjugate area of 



