DRIFTING LIGHT- WA VES. 87 



ten equal parts, or take such parts from a well-divided rule ; 

 divide one of these tenths into ten equal parts, as nearly as 

 the eye will permit you to judge ; then one of these parts, or 

 about half the thickness of an average pin, would contain 

 475 of the waves of pure green light. The same length 

 would equal the length of 440 waves of pure yellow light, 

 and of 5 1 1 waves of pure blue light. (The green, yellow, 

 and blue, here spoken of, are understood to be of the 

 precise colour of the middle of the green, yellow, and blue 

 parts of the spectrum.) Thus the green waves must be 

 increased in the proportion of 475 to 440 to give yellow 

 light, or reduced in the proportion of 511 to 475 to give 

 blue light. For the first purpose, the velocity of recession 

 must bear to the velocity of light the proportion which 30 

 bears to 475, or must be equal to rather more than one- 

 sixteenth part of the velocity of light say 1 1,600 miles per 

 second. For the second purpose, the velocity of approach 

 must bear to the velocity of light the proportion which 

 36 bears to 475, or must be nearly equal to one-thirteenth 

 part of the velocity of light say 14,300 miles per second. 

 But the motions of the stars and other celestial bodies, and 

 also the motions of matter in the sun, and so forth, are very 

 much less than these. Except in the case of one or two 

 comets (and always dismissing from consideration the 

 amazing apparent velocities with which comets' tails seem to 

 be formed), we may take too miles per second as the 

 extreme limit of velocity with which we have to deal, in 

 considering the application of our theory to the motions of 

 recession and approach of celestial bodies. Thus in the 

 case of recession the greatest possible change of colour in 

 pure green light would be equivalent to the difference 

 between the medium green of the spectrum, and the colour 

 i-n6th part of the way from medium green to medium 

 yellow ; and in the case of approach, the change would cor- 

 respond to the difference between the medium green and 

 the colour i-i43rd part of the way from medium green to 

 medium blue. Let any one look at a spectrum of fair 



