ELECTRONIC MAGNITUDES 319 



The sources of Fig. 43 may be secondary sources, 

 all receiving energy from some distant source. If they 

 are excited by monochromatic light its frequency may 

 be determined, provided the separation of the sources 

 Si, s 2 , the distance Si a and the separation of the bands 

 are measurable. This principle has been applied to 

 the measurement of the frequency of visible light by 

 using the so-called diffraction grating, a smooth surface 

 of speculum metal upon which are ruled equidistant 

 scratches. When this is illuminated by a beam of 

 light, the intervening spaces act by reflection as sources. 

 These sources are lines and not points and result in 

 spectral bands parallel to themselves, as may be seen 

 by considering the sources of Fig. 43 to be points in a 

 cross section of such a grating. The grating thus 

 serves to resolve light which is not monochromatic into 

 its monochromatic components, since the location of 

 each band will depend upon its frequency. 



Conversely, if the primary source is monochromatic, 

 an indication of the spacing of the point sources is 

 obtained from the separation of the bands. If these 

 point sources do not lie in the same line, as we have 

 so far assumed, but are in three dimensions, the result- 

 ing interference bands will form a complicated pattern 

 from which, however, some indication of the configura- 

 tion of the sources may be obtained. Of course, the 

 spacings of the sources must be regular, and not hap- 

 hazard, if this is to be done. Such a regular structure 

 is afforded by the atoms of crystalline substances, but 

 their separations are very small and a diffraction pat- 

 tern is impossible, unless the wave length of the light 

 which is used is smaller than that of visible light. In 



