644 ABBE DIFFRACTION THEORY [Cn. XV 



DEMONSTRATION OF ABBE DIFFRACTION THEORY OF MICROSCOPIC 



VISION 



909. Apparatus. Condenser; Pinhole; Slit. 



Convex lens of one meter focal length (spectacle lens of i diopter, 

 356a). 



Grating, photographic line screen (100 to 200 lines to the inch,) 

 fine wire gauze (100 mesh), fine bolting cloth. 



Telescope. The eye-piece should be of high power. 



Diaphragms to remove portions of the diffraction image. 



FIG. 372. LENS SYSTEM AND ARRANGEMENT FOR SHOWING THE ABBE 

 DIFFRACTION THEORY OF IMAGE FORMATION. 



L Right-angled arc lamp with small carbons (5 mm.) 

 C Condenser used temporarily for focusing. 



C Grating with coarse lines. A halftoning, line screen or a fine wire net will 

 answer. 



Spectacle lens of I diopter (i meter focus) for projecting the image of the 

 lamp. 



DI Image of the arc lamp L, projected by the objective O. 



When the grating (G) is in place, there is formed at this point a diffraction 

 pattern. Various shaped diaphragms placed at this point modify the screen 

 image of the grating at I. 



1 Screen image of the grating (G). 



The lines and numbers above and below indicate the approximate distances 

 between the different parts of the system which have been found to give satis- 

 factory results. 



An interesting phenomenon connected with the Abbe Diffraction 

 Theory of image formation can be demonstrated by one of the 

 combinations described below. The simplest is shown in fig. 372. 

 Suppose the test object to be a diffraction grating with equidistant 

 lines such as a fine wire gauze or a line screen such as photo- 

 engravers use in making half tones. 



910. The Abbe diffraction theory. Image formation with 

 dkected light. In microscopic work and with all transparency 



