CH. XV] ABBE DIFFRACTION THEORY 645 



projection, objects are not self-luminous but are illuminated by a 

 narrow beam of directed light which, were no object present, would 

 pass through the center of the objective. When an object, a 

 diffraction grating for example, is illuminated with a narrow cone 

 of light, the light is spread out into a diffraction pattern. The 

 finer the details of the object, the larger will be the diffraction 

 pattern. The objective will unite the light scattered from the 

 object by diffraction just as it would light which was spread out by 

 reflection from a white surface. Now according to the Abbe 

 diffraction theory, the closeness with which the image will corre- 

 spond to the object will depend upon the completeness with which 

 the light from the entire diffraction pattern is collected to form the 

 image. If the entire diffraction pattern is not united to form the 

 image, but part of it is intercepted, the image will be that of such 

 an object as would produce a diffraction pattern like that part of 

 the diffraction pattern which is collected to form the image. 



911. Lens system for showing diffraction images. The lens 

 system shown in figure 372 will show this phenomenon. The arc 

 lamp L, with 5 mm. carbons, three to five amperes direct current 

 used as a point source, is set up six to eight meters from the screen 

 at I. The condenser C is used temporarily to illuminate the 

 objective lens 0. This objective lens O, is of one meter focal 

 length (an ordinary convex spectacle lens of i diopter will answer). 

 It is placed 3M. (10 ft.) from the screen. The grating G, is placed 

 between the source L, and the objective O, so as to be in focus on the 

 screen at I . The condenser C is now removed. The image I, will 

 remain as before. At DI, would be found an image of the source 

 cast by the lens O, but it will be spread out into a diffraction pat- 

 tern by the grating. 



If a vertical slit is placed at D I, so as to remove all but a vertical 

 line of images, the appearance will be of parallel horizontal lines. 

 A diagonal slit will give the appearance of diagonal lines, no vertical 

 or horizontal lines being seen. If a vertical rod is put in so as to 

 remove the central row of images, the diffraction pattern will be 

 that of a grating with fine vertical lines, twice as close together as 

 the coarse horizontal lines, and the image at I, will have heavy 



