THE EYE AND THE PHYSIOLOGY OF VISION. 533 



to pass the added distance of the earth's orbit. In other 

 words, it took a ray of light a little over sixteen minutes to 

 cross the earth's orbit, a distance of some one hundred and 

 eighty million miles. This makes the distance traveled per 

 second about 186,000 miles. 



Thus, a ray of light would have a sufficient speed to travel 

 around the earth's surface about seven or eight times be- 

 tween two successive ticks of a second hand. 



3. The Number of Vibrations in Waves of Light. If 

 light is a wave motion the problem presents itself of having 

 the number of vibrations per second measured. Without 

 going into the physical explanation of the manner in which 

 this is accomplished, save the statement that such measure- 

 ments have been made and the number of vibrations per 

 second determined with relative accuracy, the interesting 

 fact at once comes to light that the number of vibrations 

 determines the color of the light just as in the case of 

 sound the number of vibrations determines the pitch. The 

 pitch of a light is its color. The lowest pitch in light 

 visible to the eye is red; the highest pitch is violet. When 

 the vibrations become less frequent than those in red or 

 more numerous than those in violet they do not affect the 

 eye and seem like perfect darkness. This darkness below 

 the red and above the violet is, however, not a physical 

 darkness; it is a physiological darkness. Even though the 

 etherial vibrations are there the physiological capacity of 

 the eye is not large enough to include them. 



Using approximate figures only, there are in red light 

 400,000,000,000,000 per second; in violet, 800,000,000,- 

 000,000. The lowest base note in light consists of four 

 hundred trillions ; the highest note of eight hundred trillions 

 per second. Between these two extreme pitches of color 

 are the other colors of the spectrum. Thus green, which 

 is about half way between the red and violet, has in the 

 neighborhood of six hundred trillions. 



4. The Spectrum. Ordinary sunlight, the ordinary 

 light of the day, is what we usually designate " white" 



