88 PLEASANT WA YS IN SCIENCE. 



length, or even at a correctly tinted painting of the solar 

 spectrum, and note how utterly unrecognizable to ordinary 

 vision is the difference of tint for even the twentieth part of 

 the distance between medium green and medium yellow on 

 one side or medium blue on the other, and he will recog- 

 nize how utterly hopeless it would be to attempt to appre- 

 ciate the change of colour due to the approach or recession 

 of a luminous body shining with pure green light and 

 moving at the tremendous rate of 100 miles per second. 

 It would be hopeless, even though we had the medium green 

 colour and the changed colour, either towards yellow or 

 towards blue, placed side by side for comparison how 

 much more when the changed colour would have to be 

 compared with the observer's recollection of the medium 

 colour, as seen on some other occasion ! 



But this is the least important of the difficulties affecting 

 the application of this method by noting change of colour, 

 as Doppler originally proposed. Another difficulty, which 

 seems somehow to have wholly escaped Doppler's attention, 

 renders the colour test altogether unavailable. We do not 

 get pure light from any of the celestial bodies except certain 

 gaseous clouds or nebulae. From every sun we get, as from 

 our own sun, all the colours of the rainbow. There may be 

 an excess of some colours and a deficiency of others in any 

 star, so as to give the star a tint, or even a very decided 

 colour. But even a blood-red star, or a deep-blue or violet 

 star, does not shine with pure light, for the spectroscope 

 shows that the star has other colours than those producing 

 the prevailing tint, and it is only the great excess of red 

 rays (all kinds of red, too) or of blue rays (of all kinds), and 

 so on, which makes the star appear red, or blue, and so on, 

 to the eye. By far the greater number of stars or suns 

 show all the colours of the rainbow nearly equally dis- 

 tributed, as in the case of our own sun. Now imagine for a 

 moment a white sun, which had been at rest, to begin sud- 

 denly to approach us so rapidly (travelling more than io,ooc 

 miles per second) that the red rays became orange, the 



