260 DISCOURSE ON THE STUDY 



countryman, the late Dr. Thomas Young, had esta- 

 blished a principle in optics, which, regarded as a 

 physical law, has hardly its equal for beauty, sim- 

 plicity, and extent of application, in the whole circle 

 of science. Considering the manner in which the 

 vibrations of two musical sounds arriving at once at 

 the ear affect the sense with an impression of sound 

 or silence according as they conspire or oppose 

 each other's effects, he was led to the idea that 

 the same ought to hold good with light as with 

 sound, if the theory which makes light analogous to 

 sound be the true one ; and that, therefore, two rays 

 of light, setting off from the same origin, at the same 

 instant, and arriving at the same place by different 

 routes, ought to strengthen or wholly or partially 

 destroy each other's effects according to the differ- 

 ence in length of the routes described by them. 

 That two lights should in any circumstances com- 

 bine to produce darkness may be considered strange, 

 but is literally true ; and it had even been noticed 

 long ago as a singular and unaccountable fact by 

 Grimaldi, in his experiments on the inflection of 

 light. The experimental means by which Dr. Young 

 confirmed this principle, which is known in optics 

 by the name of the interference of the rays of light, 

 were as simple and satisfactory as the principle 

 itself is beautiful ; but the verifications of it, drawn 

 from the explanation it affords of phenomena appa- 

 rently the most remote, are still more so. Newton's 

 colours of thin films were the first phenomena to 

 which its author applied it with full success. Its 

 next remarkable application was to those of diffrac- 



