150 COLOURS OF THIN PLATES. 



These imperfections of the emission-theory are still more 

 glaring when we pass from one class of phenomena to 

 another. The phenomena of diffraction, for example, are 

 referred to principles altogether different from those which 

 seemed to be required in explaining the colours of thin 

 plates ; and the two classes of phenomena, in this way of 

 accounting for them, bore no relation of any kind to each 

 other. 



All this is otherwise in the wave-theory. Here the 

 several classes of phenomena are deduced from a common 

 principle, and are, therefore, mutually related. The prin- 

 ciple of interference is a necessary consequence of the nature 

 of a vibration ; and this one principle, as we have seen, ex- 

 plains in the most complete manner the laws of the other 

 phenomena. 



But it is not merely in their reference to a common 

 origin that these phenomena are thus related : they are 

 even connected numerically. The simple laws of inter- 

 ference, the laws of diffraction, and those of thin plates, 

 are all dependent upon a single constant for each kind of 

 light,^-the length of a wave in each medium; and this 

 constant being inferred from any one experiment, in any 

 one class of phenomena, we can compute numerically the de- 

 tails of all the rest, and compare them with the results of 

 measurement. The agreement is found to be complete. 



