DRIFTING LIGHT- WA YES. 



319 



sound of one determinate note. As the aerial 

 waves producing the effect of one definite tone 

 are all of one length, so the ethereal waves pro- 

 ducing light of one definite color are all of one 

 length. Therefore, if we approach or recede 

 from a source of light emitting such waves, 

 effects will result corresponding with what has 

 been described above for the case of water-waves 

 and sound-waves. If we approach the source of 

 light, or if it approaches us, the waves will be 

 shortened ; if we recede from it, or if it recedes 

 from us, the waves will be lengthened. But the 

 color of light depends on its wave-length pre- 

 cisely as the tone of sound depends on its wave- 

 length. The waves producing red light are longer 

 than those producing orange light, these are 

 longer than the waves producing yellow light ; 

 and so the length-waves shorten down from yel- 

 low to green, thence to blue, to indigo, and 

 finally to violet. Thus if light shining in reality 

 with a pure green color approached the observer 

 with a velocity comparable with that of light, it 

 would seem blue, indigo, or violet, according to 

 the rate of approach; whereas if it rapidly re- 

 ceded, it would seem yellow, orange, or red, ac- 

 cording to the rate of recession. 



Unfortunately in one sense, though very for- 

 tunately in many much more important respects, 

 the rates of motion among the celestial bodies 

 are not comparable with the velocity of light, 

 but are always so much less as to be almost rest 

 by comparison. The velocity of light is about 

 187,000 miles per second, or, according to the 

 measures of the solar system at present in vogue 

 (which will shortly have to give place to some- 

 what larger measures, the result of observations 

 made upon the recent transit of Venus), about 

 1S5,000 miles per second. The swiftest celestial 

 motion of which we have ever had direct evi- 

 dence was that of the comet of the year 1843, 

 which, at the time of its nearest approach to the 

 sun, was traveling at the rate of about 350 miles 

 per second. This, compared with the velocity 

 of light, is as the motion of a person taking six 

 steps a minute, each less than half a yard long, 

 to the rush of the swiftest express-train. No 

 body within our solar system can travel faster 

 than this, the motion of a body falling upon the 

 sun from an infinite distance being only about 

 370 miles per second when it reaches his sur- 

 face. And though swifter motions probably exist 

 among the bodies traveling around more mas- 

 sive suns than ours, yet of such motions we can 

 never become cognizant. All the motions taking 

 place among the stars themselves would appear 



to be very much less in amount. The most swiftly 

 moving sun seems to travel but at the rate of 

 about 50 or 60 miles per second. 



Now let us consider how far a motion of 100 

 miles per second might be expected to modify 

 the color of pure green light — selecting green as 

 the middle color of the spectrum. The waves 

 producing green light are of such a length, that 

 47,000 of them scarcely equal in length a single 

 inch. Draw on paper an inch and divide it care- 

 fully into 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 

 511 waves of pure blue light. (The green, yel- 

 low, and blue, here spoken of, are understood to 

 be of the precise color 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 11,600 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 ve- 

 locity of light — say 14,300 miles per second. 

 But the motions of the stars and other celes- 

 tial 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 consid- 

 eration the amazing apparent velocities with 

 which comets' tails seem to be formed), we may 

 take 100 miles per second as the extreme limit 

 of velocity with which we have to deal in con- 

 sidering the application of our theory to the mo- 

 tions of recession and approach of celestial bod- 

 ies. Thus, in the case of recession the greatest 

 possible change of color in pure green light would 

 be equivalent to the difference between the me- 

 dium green of the spectrum, and the color -\-$ 

 part of the way from medium green to medium 

 yellow ; and in the case of approach, the change 

 would correspond to the difference between the 

 medium green and the color -\ ri part of the way 

 from medium green to medium blue. Let any 



