542 THE POPULAR SCIENCE MONTHLY. 



those of the grave sounds ; for they all have the same velocity of 

 progress, and reach us in the same time. The melody and har- 

 mony are heard simultaneously, whatever the distance of the 

 orchestra. The exact sensation of the piece played is felt on 

 every side a thing which could not take place if the high tones 

 of the violins and flutes were transmitted more rapidly than the 

 grave sounds of the violoncellos and contrabasses. It being 

 thus possible to assimilate simple sounds with simple colors, we 

 have to suppose that the number of vibrations determines the 

 color. A luminous point produces, to emit the various colors : 

 red, 497 ; orange, 528 ; yellow, 529 ; green, 601 ; blue, G48 ; indigo, 

 686; and violet, 728 trillion vibrations per second. Each color 

 corresponds with a luminous film of variable thickness. The 

 thicknesses of the several films representing the simple colors or, 

 what are the same, the wave-lengths of these colors are: red, 

 620 ; orange, 583 ; yellow, 551 ; green, 512 ; blue, 475 ; indigo, 449 ; 

 violet, 423 millionths of a millimetre. Red, we thus see, corre- 

 sponds to the grave notes and violet to the acute notes of the 

 musical scale. To obtain an idea of the thickness of the films 

 corresponding to the different colors, we might take as a standard 

 for comparison a sheet of common paper, which is about a tenth 

 of a millimetre thick. Two hundred and fifty thicknesses of the 

 violet film would have to be laid upon one another to produce 

 this thickness, and one hundred and sixty of the red. 



In order to explain the cause of the complex colors of natural 

 objects we may again have recourse to the properties of vibrating 

 motions, which, like those of the phenomena of sound, can be 

 placed one upon another. Thus, when a cord is stretched over a 

 sonorous box, like the string of a violoncello, we can make it all 

 vibrate ; its ends will be motionless, while the middle will vibrate 

 with the maximum amplitude. The motionless extremities are 

 called nodes, and the middle is a belly. We can also draw the 

 bow across this cord in such a manner that, while vibrating as a 

 whole, the two halves of the cord will each vibrate on its own ac- 

 count, following a law of individual vibration. Under these con- 

 ditions a superposition of two vibratory movements is realized 

 that of the whole cord and that of the two halves vibrating sepa- 

 rately. There results a complex sound formed of the fundament- 

 al sound and the superposed harmonic. It is this superposition 

 that gives to the ear the sensation of the timbre of different 

 sounds ; the phonograph, with which everybody is acquainted, is 

 based on this principle. The vibrations of a single membrane 

 can reproduce several superposed vibratory movements, and thus 

 register human speech. 



Most of the complex colors, such as rose, maroon, or the va- 

 rious tints of green, can be formed in the same manner. They 



