THE WAVE THEORY. 269 



in the channel being the result. * At least,' says Young, 

 * I can discover no alternative, either from theory or 

 from experiment. Now,' he continues, gathering con- 

 fidence as he reasons, * I maintain that similar effects 

 take place whenever two portions of light are thus 

 mixed, and this I call the general law of the Inter- 

 ference of Light.' 



The physical meaning of all the terms applied to 

 light was soon fixed. Intensity depended upon the 

 amplitudes of the waves. Colour depended on the 

 lengths of the waves. Two series of waves coalesced 

 and helped each other when one was any number of 

 complete undulations, or, in other words, any even 

 number of half-undulations, behind the other. Two 

 series of waves extinguished each other when the one 

 series was any odd number of semi-undulations behind 

 the other. But inasmuch as white light is made up of 

 innumerable waves of different lengths, such waves 

 cannot all interfere at the same time. Some interfere 

 totally, and destroy each other ; some partially ; while 

 some add themselves together and enhance the effect. 

 Thus, by interference, a portion only of the white light 

 is destroyed, and the remaining portion is, as a general 

 rule, coloured. Indeed most glowing and brilliant 

 effects of coloration are thus produced. Young applied 

 the theory successfully to explain the colours of striated 

 surfaces which, in the hands of Mr. Eutherfurd and 

 others, have been made to produce such splendid effects. 

 The iridescences on the polished surfaces of mother- 

 of-pearl are due to the striae produced by the edges of 

 the shell-layers, which are of infinitesimal thickness; 

 the fine lines dnovn by Coventry, Wollaston, and Barton 

 upon glass also showed these colours. Barton after- 

 wards succeeded in transferring the lines to steel and 

 braes. Most of you are acquainted with the iridescence 



