THE MOVEMENTS OF THE STARS. 275 



red light, while vibrations of other numbers per second 

 correspond to the orange and blue rays respectively. We 

 must, therefore, understand that our sense of vision dis- 

 tinguishes between one colour and another solely by the 

 different number of vibrations of the ether to which the 

 rays correspond. On the correct apprehension of this 

 principle depends the right understanding of the spectro- 

 scopic method for the detection of motion along the line 

 of sight. 



We have been hitherto supposing that the body from 

 which the light emanates is at rest. Now let us suppose 

 that it is in motion to or from the observer ; we shall see 

 that this would have the effect of modifying the inter- 

 pretation which the brain puts on the sensations which 

 it receives from the retina. The subject is one of some 

 difficulty, but I hope by the help of a few illustrations to 

 make it intelligible. 



I remember some time ago hearing a question pro- 

 pounded something of this kind. Suppose that it takes a 

 week for a steamer to cross from Liverpool to New York, 

 and that one steamer starts from each end every day. Of 

 course we may suppose that, on an average, one arrives 

 every day, so that in the course of a week seven steamers 

 reach Liverpool. But now let us suppose that the ob- 

 server, instead of being stationed at Liverpool, takes his 

 berth on one of the outgoing steamers. Then the question 

 may be asked as to how many returning steamers he will 

 pass in the course of his week's voyage to New York. 

 One might hastily answer seven, that is the same number, 

 no doubt, that would be counted in a week from the 

 landing-stage at Liverpool, but the case is very different 



