124 Diffraction Experiments. 



bounded by straight lines"* The investigations of Fresnel 

 show that a strict application of the undulatory doctrine of 

 light, assisted by the principle of interferences, will be found 

 to afford a full and precise explanation of all the facts, regard- 

 ing the opaque body merely as an obstacle bounding the waves 

 propagated from the luminous point on one side."t 



Let us now endeavour by appropriate analogies to obtain an 

 elementary conception of light as a system of vibrations of a 

 subtle fluid, or ether, and therefore bearing a certain resem- 

 blance to sound, or to water waves. It is easy to realize the 

 fact that sound consists in vibrations and waves of air. The 

 large strings of a harp or guitar may be seen to move when 

 agitated so as to produce sounds, and various contrivances 

 have been devised to make phenomena of this kind strikingly 

 appreciable by the eye. When a stone is thrown into a still 

 pond we have another illustration of wave forms, and we see 

 them propagated in all directions, in circles increasing in di- 

 mensions, until the margin is reached. The most violent of 

 these waves are those nearest to the stone causing the disturb- 

 ance, and as they grow wider and act upon a greater breadth of 

 water, they grow feebler likewise, until at last, if the pond be 

 big enough, they die away in ripples too faint for observation. 

 If, after we have set one series of waves in action, we start 

 another with a different velocity, it will happen that some of 

 the hollows of the first set will coincide more or less completely 

 with the elevations of the second set ; and when this occurs, a 

 more or less perfect calm ensues, while the addition of hollow 

 to hollow, or crest to crest, deepens the disturbance, and mag- 

 nifies the miniature storm. If waves of sound are started one 

 after another in due proportion, or if two sound waves, with 

 just the right difference between them, be started together, 

 this process of partial filling each other up may be produced, 

 and at certain points of the wave march, corresponding with 

 definite intervals of time, silence will result from the concur- 

 rence of two sounds. If at certain intervals the hollow of one 

 wave is exactly filled up by the crest of another, no sound will 

 be heard ; but a partial filling up will modify the sound, and 

 make it yield a different note. Much of the art of music con- 

 sists in the production of well-managed interferences, giving 

 rise to varied and highly pleasing effects. 



If we pass from sound to light, we find ourselves able to 

 produce bands of darkness by the interference of two rays of 

 light. We can also devise methods of interference that 

 shall give rise to modified waves having the dimensions neces- 

 sary to excite upon our eyes the effects belonging to all the 



* 8ir J. HerBchel's Treatise on Light, p. 480. t P. 481. 



