232 
DR. J. YOUNG AND PROFESSOR G. FORBES ON THE 
plished by the experiments conducted in 1880-81 between Kelly House, Wemyss 
Bay, and the hills behind Innellan, across the mouth of the River Clyde. 
The chief importance of a determination of the velocity of light is that it gives 
us the means, considered by many to be the best means, of determining the solar 
parallax, by combining the result with the constant of aberration determined by 
astronomers. The investigation has also acquired a further interest from the specu¬ 
lations of the late Professor Clerk Maxwell, according to which the propagation 
of light is an electro-magnetic phenomenon, and its velocity should be the same as 
that of the propagation of an electro-magnetic displacement. 
Our researches have, however, led us to results of great importance in another 
direction. We find reasons for believing that the different colours of which white 
light is composed do not travel with the same velocity, bub that the more refrangible 
rays travel more rapidly through a vacuum, and that this difference is by no means 
very small, so that we may expect its presence to be determined by independent 
tests. The influence of this conclusion upon our views about the constitution of the 
ether is considerable, and will doubtless lead to some further knowledge of its 
properties. 
The general theory of the method employed in the present research resembles that 
of M. Fizeau. In his experiments, and in those of M. Cornu, a beam of light is 
sent through between the teeth of a toothed wheel to a distance of some miles, 
whence it is reflected back again by means of certain optical arrangements, which 
bring it back to the same point. If the wheel be rotating at a suitable rate, then, 
by the time that the light has gone to the distant station and back, the position of 
the space between two teeth is now occupied by a tooth and the light cannot pass. 
But if the velocity of the toothed wheel be doubled that position will be occupied by 
the next space and the light is able to pass. If the velocity be increased threefold, 
fourfold, &c., we have alternately eclipses and full brightness. 
If m be the number of teeth in the wheel, it must, in the time that light has gone 
to the distant station and back again, have completed of a revolution in order to 
bring a tooth into the position previously occupied by a space at the commencement 
of that time. If the wheel be found to be making N revolutions a second at the time 
of the first eclipse, then —— is the fraction of a second taken by fight to perform the 
double journey; and if the distance between the toothed wheel and the distant reflector 
be D then the velocity of fight is 
V = 4mND. 
In the method which we devised, instead of having only one distant reflector, we 
have two, nearly in the same fine, but one of them being at a greater distance than 
the other and a little to one side of it, Let us call the most distant reflector A and 
