278 IN STARRY REALMS. 



case if the two bodies were relatively stationary. But 

 we have already seen that our senses have no means 

 of interpreting the hue of any beam that enters the eye 

 except by the number of undulations per second that it 

 causes on the retina. It follows that in the case we have 

 supposed, when the eye received more vibrations per 

 second than was appropriate to that particular shade of 

 green, the appearance presented to us would be that the 

 star had a different hue from that which it actually pos- 

 sessed it would, in fact, shine with a shade of colour 

 nearer the blue end of the spectrum. Indeed, if the speed 

 of approach were sufficiently great the star would cease 

 to appear green at all, and would manifest a bluish hue 

 and if we allow sufficient scope to our imagination in the 

 matter of velocity, we could conceive a star approaching 

 so fast that its colour, though originally green, appeared 

 to be indigo or even violet, while by a still farther stretch 

 we might suppose that the star is dashing towards us 

 with a pace even greater than that which would trans- 

 form its original green to an apparent violet. The 

 vibrations would then be poured into the eye with such 

 rapidity that no nerves on the retina, constructed as it is, 

 would be adapted to respond to them, and consequently 

 the star would become actually invisible to us. 



On the other hand, let us suppose that this green-coloured 

 monochromatic star was dashing away from the earth with 

 sufficient speed, or, what comes to the same thing, that we 

 were dashing away from it then a transformation in the 

 apparent character of the light would take place in the 

 opposite direction. The shift of the green colour would be, 

 in this case, not towards the blue end of the spectrum but 



