CLASSICAL THEORY OF LIGHT 749 



perfectly "amorphous" substance; perhaps not even liquids satisfy 

 this requirement. 



Interference 



When a pair of beams of light are projected together upon a screen, 

 it is usually observed that the illumination resulting from them 

 jointly is the simple sum of the illuminations which each produces by 

 itself when the other is shut off. One may easily go through life 

 without ever once finding this rule in default. Yet by intelligent 

 design it is possible to contrive conditions in which the rule does not 

 prevail; and actually two rays of light directed upon the same point 

 may counteract one another and cause total darkness, and two 

 perfectly uniform wide beams falling together upon a surface of frosted 

 glass may decorate it with a pattern of dark fringes separated by 

 light, dark circles alternated with bright, black networks upon a back- 

 ground of color — arabesques of shadow and light, more delicately 

 shaded than anything achievable in pigment or stained glass. The 

 brilliant and versatile Thomas Young, he who was the first to read the 

 Egyptian hieroglyphics upon the Rosetta stone, was also the first to 

 discover some of these lovely phenomena; a pair of exploits, which 

 for eminence and diversity will probably never be surpassed. It 

 happened that the first disclosure of the phenomena which demand the 

 wave-theory of light coincided as accurately with the advent of the 

 nineteenth century as the first realization of the necessity of quanta 

 came at the dawn of the twentieth; for Young discovered the inter- 

 ference of light in 1800. 



"Interference" is a name which Young selected; he said that in 

 the conditions of his experiments beams of light interfere with one 

 another. For the observer this was not, on the whole, an ill-chosen 

 word, since the visible effect of the two lights conjointly is not the 

 mere sum of the visible effects of each separately. True, it implies 

 that the lights destroy or diminish one another, whereas in fact they 

 are as likely to cooperate as to conflict, two equal beams combining 

 into one of intensity as much as fourfold that of either. This is not 

 serious; we are all accustomed to using the word addition to cover 

 subtraction; and here the analogy is very close. The so-called "inter- 

 ference" is simply the necessary result of adding two vibrations with 

 due regard to their direction and their phase. This is the method which 

 was used to calculate diffraction-patterns; and in fact a diffraction- 

 pattern is nothing but a special case of interference-pattern — not 

 usually a simple one, for the vibrations which must be summed are 

 very numerous, demanding integrations and long summations. The 

 simplest interference-pattern occurs when two plane-parallel beams 



