THE MOVEMENTS OF THE STARS. 277 



period. If, however, the two bodies are retreating, then 

 fewer waves per second will enter the eye than in the 

 former case. We have, however, already seen that the 

 colour of the light depends on the number of undulations 

 in a second which strike the retina ; if this number be 

 altered, then the colour of the light, at least as interpreted 

 by our senses, will be altered also ; this principle leads to 

 some curious consequences, and in order to explain the 

 matter fully it will be necessary for me to take a some- 

 what simpler case than any that actually occurs in nature. 



Generally speaking, the light radiated from a star or 

 from any other luminous source is of a highly composite 

 character, consisting of a number of undulations of very 

 varied wave lengths in fact, of a multitude of hues 

 blended together. Thus, in the case of the sun, as well as 

 in most of the stars, the light received from them consists 

 of hues of all the colours of the rainbow, as well as of many 

 rays which are invisible, because they vibrate too quickly 

 in regions beyond the violet, or because they vibrate too 

 slowly for our eyes to recognise in regions beyond the 

 red. Let us, however, for the moment assume that a 

 star existed which dispensed rays of a perfectly uniform 

 type, which it will be convenient to take at a central 

 part of the spectrum, say in the position of the green. 

 Let our supposed star shed forth a flood of undulations all 

 of precisely similar wave length, and let us study the 

 modifications that would be caused in the appearance of 

 that star if it moved rapidly towards the earth. 



According to the principle we have already explained, 

 as the two bodies come together, more undulations 

 would enter the eye in a second than would be the 



