86 PLEASANT WA YS IN SCIENCE. 



in reality with a pure green colour, 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 receded, it would seem yellow, orange, 

 or red, according to the rate of recession. 



Unfortunately in one sense, though very fortunately 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 somewhat larger measures, the result of observa- 

 tions made upon the recent transit of Venus), about 185,000 

 miles per second. The swiftest celestial motion of which we 

 have ever had direct evidence was that of the comet of the 

 year 1843, which, at the time of its nearest approach to the 

 sun, was travelling 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 dis- 

 tance being only about 370 miles per second when it reaches 

 his surface. And though swifter motions probably exist 

 among the bodies travelling around more massive 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 colour of pure 

 green light selecting green as the middle colour 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 carefully into 



