Thus was discovered the physical cause of the splendor and variety of colors, 

 and a singular and mysterious alliance was developed between color and sound. 

 T.icrhts are f various hues, according to the magnitude of the pulsations that 

 prom: t them, exactly as musical sounds vary their tone and pitch according 

 to the magnitude of the aerial pulsations from which they result. 



But this is not all. The alliance between sound and light does not termi- 

 nate here. We have only spoken of the amplitude of the luminous waves, and 

 have shown that it determines the tints of colors. What are we to say for the 

 altitudes of the waves 1 Here, again, is another link of kindred between the 

 eye and the ear. As the altitude of sonorous waves determines the loudness 

 of the sounds, so the altitude of luminous waves determines the intensity or 

 brightness of the color. 



There is one step more in the series of wondrous results which these mem- 

 orable investigations have unfolded. As the perception of sound is produced 

 by the tympanum of the ear vibrating in sympathetic accordance with the pul- 

 sations of the air produced by the sounding body, so the perception of light and 

 color is produced by similar pulsations of the membrane of the eye vibrating 

 in accordance with ethereal pulsations propagated from the visible object. As 

 in the case of the ear, the rigor of scientific investigation requires us to estimate 

 the rate of the pulsation of the tympanum corresponding to each particular note, 

 so in the case of light are we required to count the vibrations of the retina of 

 the eye corresponding to every tint and color. It may well be asked, in some 

 spirit of incredulity, how the solution of such a problem could be hoped for ; 

 yet, as we shall now see, nothing can be more simple and obvious. 



Let us suppose an object of any particular color, as a red star, for example, 

 placed at a distance and seen by the eye. From the star to the eye there pro- 

 ceeds a continuous line of waves ; these waves enter the pupil and impinge 

 upon the retina ; for each wave which thus strikes the retina, there will be a 

 separate pulsation of that membrane. Its rate of pulsation, or the number of 

 vibrations which it makes per second, will therefore be known, if we can as- 

 certain how many luminous waves enter the eye per second. 



It has been already shown that light moves at the rate of about two hundred 

 thousand miles per second ; it follows, therefore, that a length of ray amount- 

 ing to two hundred thousand miles must enter the pupil each second ; the num- 

 ber of times, therefore, per second, which the retina will vibrate, will be the 

 same as the number of the luminous waves contained in a ray two hundred 

 thousand miles long. 



Let us take the case of red light. In two hundred thousand miles there are 

 in round numbers a thousand millions of feet, and therefore twelve thousand 

 millions of inches. In each of these twelve thousand millions of inches there 

 are forty thousand waves of red light. In the whole length of the ray, therefore, 

 there are four hundred and eighty millions of millions of waves. Since this 

 ray, however, enters the eye in one second, the retina must pulsate once for 

 each of these waves ; and thus we arrive at the astounding conclusion, that 

 when we behold a red object, the membrane of the eye trembles at the rate of 

 four hundred and eighty millions of millions of times between every two ticks 

 of a common clock ! 



In the same manner, the rate of pulsation of the retina corresponding to other 



' tints of colors is determined ; and it is found that when violet light is perceived, 



| it trembles at the rate of seven hundred and twenty millions of millions of times 



> per second. 



In the annexed table are given the magnitudes of the luminous waves of each 



color, the number of them which measure an inch, and the number of undula- / 



[ tions per second which strike the eye : 



