-1859] WRITINGS OF JOSEPH HENRY. 101 



a distance of half an undulation, they neutralize each other, 

 the protuberance of the one undulation exactly filling as 

 it were the hollow of the other; or to express it more accu- 

 rately, the rarefied and condensed parts of the two waves 

 will neutralize each other, and in this way silence may be 

 produced by two intense sounds. From analogy therefore, 

 if light also consists of waves, two series might be brought 

 together, so as to produce darkness. Both these inferences 

 are fully borne out by experiment. 



If we observe the effect of the sound waves upon a 

 distant object, (such for instance as a delicate membrane 

 stretched over a hoop and strewed with sand,) we shall find 

 that on sounding an instrument the sand will be violently agi- 

 tated : and if the vibration is in unison with any of the strings 

 of a neighboring piano, they will give forth an audible sound. 



It may be well to stop one moment to inquire in what 

 this unison consists. It is well known that a string of a 

 given length performs all its vibrations in the same time. 

 Now if the impulses from the sounding body reach a string 

 of such a time of vibration that the effect of the second im- 

 pulse may be added to that of the first, or while the string 

 is moving in the same direction as that given it by the first 

 impulse, then the sounding will take place, or the string will 

 be aroused into a motion harmonious with that of the sound- 

 ing body. But if the impulses are not timed exactly to the 

 vibrations of the string, they will meet the latter in its for- 

 ward as well as in its backward movement, and thus tend 

 to neutralize the effects of each other. 



In the case of light and heat, the luminous or heated body 

 is supposed to be in the condition of the bell during its 

 sounding. The setherial medium is the analogue of the air, 

 and the vibrations of the optic nerve that of the tympanum 

 of the ear. 



Further, in the case of heat, when the vibrations from 

 the sun impinge upon the surfaces of solids and liquids, 

 the setherial medium within the interstices of these bodies, 

 and also the atoms of gross matter, are put in a state of har- 

 monious vibration, and thus give rise to the phenomena 



