LIGHT. 



Let us suppose an object of any particular colour, a red star, 

 for example, looked at 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, therefore, 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 miles long. 



Let us take the case of red light. In 200,000 miles there are 

 in round numbers 1000,000000 feet, and therefore 12000,000000 

 inches. In each of these 12000,000000 of inches there are 

 40000 waves of red light. In the whole length of the ray, 

 therefore, there are 480,000000,000000 waves. Since this ray, 

 however, enters the eye in one second, and the retina must 

 pulsate once for each of these waves, we arrive at the 

 astounding conclusion, that when we behold a red object, the 

 membrane of the eye trembles at the rate of 480,000000,000000 

 of times between every two ticks of a common clock ! 



In the same manner, the rate of pulsation of the retina corre- 

 sponding to other tints of colours is determined ; and it is found 

 that when violet light is perceived, it trembles at the rate of 

 720,000000,000000 of times per second. 



In the annexed table are given the magnitudes of the luminous 

 waves of each colour, the number of them which measure an inch, 

 and the number of undulations per second which strike the eye : 



The preceding calculations are, as will be easily perceived, 

 made only in round numbers, with a view of rendering the 

 principles of the investigation intelligible. In the table the 

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