Colours of Stars. Ill 



light to the receptive power of our nervous system, with a 

 varied organization it might not be impossible to perceive hues 

 different from those to which alone our optic nerve is capable 

 of responding. But we are wandering too far into the regions 

 of mere speculation, and in some danger of losing sight of our 

 original subject, which was not so much the colour of stellar 

 light in general, as its liability to change. It would be pre- 

 mature to conjecture whether, supposing that change established 

 as a fact, spectrum analysis may be able to give us any infor- 

 mation as to its cause ; but there seems little expectation from 

 any other quarter. One curious hypothesis however deserves 

 to be mentioned from its ingenuity rather than its verisimilitude. 

 It has been suggested by Sestini that the components of 

 double stars may change their colours periodically, in conse- 

 quence of the corresponding alteration in the direction of their 

 orbital velocity towards or from our eye. For since it is sup- 

 posed that colour consists of very minute undulations or vibra- 

 tions, and that these are much more rapid at the violet than the 

 red end of the spectrum, it would follow that any change in the 

 rate of the vibrations would be attended with a corresponding 

 alteration of colour. Now it is evident that the motion of the 

 radiant body towards or from the eye, provided that it is swift 

 enough to bear a certain proportion to the velocity of light, 

 must produce an apparent increase or diminution in the rapidity 

 of the vibrations, since, though their actual rate remains un- 

 changed, more or fewer of them will reach the eye in the same 

 time, according as the motion of their source concurs with or 

 is opposite to that of light ; and hence would result a corres- 

 ponding change of colour. There can be no doubt as to the 

 correctness of this inference, which is fully borne out by a 

 curious analogy. It has been pointed out by Mr. Scott Russell 

 that in railway travelling the pitch of sounds towards which 

 we are rapidly approaching — for instance, the whistle of an 

 engine coming in the contrary direction — will be raised or 

 sharpened from an increased number of the vibrations which 

 constitute sound entering the ear in the same time ; at the 

 moment of nearest approach the sound will descend to its true 

 pitch, and subsequently grow flatter as the distance widens 

 again. A little attention will enable us to try for ourselves 

 this interesting experiment, which presents, in regard to 

 another of our senses, a perfect correspondence with Sestini's 

 idea as to colour. But notwithstanding this collateral support, 

 his hypothesis is open to grave objection. The velocity of 

 railway transit bears a much larger proportion to that of sound, 

 than any probable orbital velocity of a star does to that of 

 light, for, as Smyth has remarked, though we have no right to 

 assume that the velocity of stellar is identical with that of 



