VELOCITY OF WHITE AND OF COLOUEED LIGHT. 
289 
in full splendour should be red. If we could determine the difference in time between 
the time of maximum for blue and for red rays, the present research would enable us 
to determine approximately the distance of the star from us. 
Variable stars are certainly known to change colour; but the only reference to a 
systematic law in this connexion which we have ever come upon is in Webb’s 
‘Celestial Objects for Common Telescopes,’ where (Edition II. (1866), p. 208) the 
following remarkable passage occurs :— 
“Hind thinks several variable stars increase blue, are yellow after maximum, and 
flash red in decreasing.” 
This is in exact accordance with the results of the present research. 
We have applied to Mr. Hind for details of his observations. He informs us that 
the remark quoted by Webb was made in a letter to the “ Times,” some twenty-six 
years since, on the occasion of notifying a newly discovered variable. He does not, 
however, feel justified in now advising us to place reliance on this as a law. 
It appears, then, that our conclusions as to the relation between colour and velocity 
must for the present rest upon the merit of our observations. We have ourselves no 
hesitation in saying that the effects were so striking and so decided on the greatest 
number of occasions, that we ourselves have no doubt as to the general conclusion. 
But we admit that much is still left to be done in the way of absolute determinations. 
There is little doubt that a further investigation into the matter would help to a 
further knowledge of the properties of the ether (if there be such a substance). This 
end would be specially aided by an exact determination of the law of dependence of 
velocity upon wave-length. We believe that, with certain improvements in our 
apparatus, in a better climate, this determination might be completed.* 
* The details of our observations on the dependence of velocity on colour are deposited with the Eoyal 
Society. 
2 p 
MDCCCLXXXII, 
