THE ANALYSIS OF LIGHT. 339 



tion of any particular color has been determined, and the length of the waves 

 corresponding to these vibrations. 



What relation The waves Tequisitc to produce red are the 

 the'^'^ wavet^" largest ; orange comes next ; then yellow, 

 br"aon?o°fM,!e S^'^^^, ^1^^, Indlgo, and violet, succeed each 

 different colors? other, thc wavcs of each being less than the 

 preceding. The rapidity of vibration is in the same order, 

 the waves producing red light vibrating with the least 

 rapidity, and the waves producing violet with the greatest 

 rapidity. 



To produce red light it is necessary that 40,000 waves or undulations should 

 be comprised within the space of a single inch, and that 480 billions of vibra- 

 tions should be executed in one second of time ; while, for the production of 

 violet, 60,000 waves within an inch, and 720 bilhons of vibrations per second 

 are required.* 



699. As two sets of sound-waves or vibra- 



Can waves of . t r- i 



light be made tious may SO combmo as to modiiy or destroy 



each other, and thus produce partial or total 



silence, so two waves or vibrations of light may be made 



to interfere and jDroduce various colors, or entire darkness. 



• It may perhaps be asked, with something of incredulity, how such a resalt could pos- 

 sibly have been arrived at, with any degree of scientific accuracy. The problem, how- 

 ever, is not a difficult one. 



In the first place, Newton, by a series of perfectly satisfactory and beautiful experi- 

 ments, ascertained the number of waves or andulations of the different colored raya 

 comprised within the space of an inch. 



Let us now suppose an object of any particular color, a red star, for example, to be 

 viewed from a distance. From the star to the eye there proceeds 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 pulsations which it makes per second, will therefore be known, 

 if we can ascertain how many luminous waves enter the eye per second. 



It has been already shown that light moves at the rate of about 200,000 miles per 

 second ; it follows, that a length of ray amounting to 200,000 miles must enter the pupil 

 each second ; the number 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 200,000 mile* 

 long. 



Let us take the case of red light In 200,000 miles there are, in round numbers, 

 3,000,000,000 feet, and therefore 12,000,000,000 inches. In each of these 12,000,000,000 of 

 inches there are 40,000 waves of red light. In the whole length of the ray, therefore, there 

 ore 4S0,u00,000,000,0O0 waves. Since this ray, however, enters the eye in one second, 

 and the retina must pulsate once for each of these wavts, we arrive at the astounding 

 conclusion, that when we behold a red object, the membrane of the oye trembles at the 

 rate of 450,000,000,000,000 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 is perceive^i it trembles at the rate 

 of 720,000,000,000,000 of times per second.— iardzwr. 



