318 THE REALITIES OF MODERN SCIENCE 



tude of the effect of these will differ from point to 

 point. Thus suppose that at a the disturbance is a 

 maximum. At some point like fr, however, it will 

 happen that the disturbance received from any source, 

 as si t will be J us t opposite in its effect to that received 

 from the adjacent source, s 2 . Similarly for other 



pairs of sources. At b there is therefore complete inter- 

 ference. At another point, as c, the effects will again 

 be in phase and there will be reenforcement and a 

 consequent maximum. The distance ac is dependent 

 upon the frequency of the several identical oscillators. 

 If these individually give rise to more than one fre- 

 quency there will be a different series of points like 

 b and c for each frequency, with the result that the 

 location of the maximum will no longer be a point but 

 will appear as a spectral band. 1 



1 The phenomenon is easily observed if one makes in a visiting 

 card two small pinholes separated by about half the diameter of a 

 pinhead. Holding this card close to the eye and observing a dis- 

 tant and intense source of light, e.g. a street lamp, the two holes 

 act like two sources in the diagram of Fig. 43 and the line ad is 

 the retina. A series of spectral bands should then be observed. 

 If it is not, the pinholes are probably too large or too far apart, or 

 both. The pattern seen through the meshes of an umbrella while 

 looking at a distant light is a familiar illustration. 



