i Light and Sound. [Jan., 



which reaching the ear give rise to the sensation of sound. When 

 a substance is made incandescent, the luminous vibrations do not 

 throw the air into undulation ; a finer and more elastic medium is 

 requisite to accept and carry on molecular motion. We have 

 abundant reason to believe that such a medium exists ; we term it 

 the luminiferous ether. To this ether, then, the luminous body 

 communicates its motion. Here, also, a system of waves is produced 

 which, striking the eye, finally give birth to the sensation of light. 

 There is, however, this difference between the sound-waves of the 

 air and the light-waves of the ether, that whereas the former 

 move to and fro longitudinally, the latter vibrate transversely. A 

 cornfield ruffled with gusts of wind exhibits waves like those of 

 sound ; the water of a lake thrown into ripples by a disturbance ex- 

 hibits waves like those of light. The upper figure, l, in the accom- 

 pany drawing (Fig. 1) shows a single plane-wave of light : the 



Fig. 1. 



lower, s, a single wave of sound. Each has at n its node, or place 

 of rest, and at v its vibrating segment, or place of greatest motion. 

 The sizes of the two waves are vastly disproportionate. The average 

 length of a sonorous wave, say that of the middle o in the piano, is 

 about 50 inches from n to n'. The average length of a luminous 

 wave, say that of green light, is only the yowWoth part of an inch 

 from n to n'. This great difference must not stagger us in tracing 

 out our analogy. The element of size does not enter into the 

 region of law. The truth of this statement will become evident as 

 we see laws obeyed in like manner by light and sound ; by undula- 

 tions, one set of which are millions of times the size of the other. 



Another strong point in this analogy is presented by the phe- 

 nomena of Interference. From an inspection of Fig. 1, it will 

 readily be seen how this is produced. Let L be a series of waves ; 

 suppose a second series, of exactly the same length as L, to start 

 just half an undulation later; it will be clear that the crest of 

 one set will coincide with the hollow of the other. They will thus 

 mutually destroy each other, and darkness will be the product of 



