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Y. Experimental Determination of the Velocity of White and of Coloured Light. 
By James Young, LL.D., F.P.S., and Professor G. Forbes. 
Received May 17,—Read May 19, 1881. 
Contents. 
Part I. —Absolute Velocity of White Light. Page. 
Introductory. 231 
Mathematical theory of our method... 233 
Description of the apparatus..... 240 
Establishment of the apparatus at Kelly, Wemyss Bay .. 249 
Measurement of the distances........ 250 
Translation of the chronograph records ... 252 
Part II. —Relative Velocity of Lights of different Colours. 
Does the velocity of light depend upon its colour P ..... 270 
Distinctive colours observed in the return light.. 272 
Explanation of these colours. 274 
First measurements with red and white light... 275 
Differential observations. 277 
PART I. 
Absolute Velocity oe White Light. 
Introductory. 
It is remarkable that, although the importance of an accurate knowledge of the 
velocity of light has been very generally appreciated, no attempt has hitherto been 
made in this country to measure that velocity by experiment. Our own experiments 
date from many years back, but we have been prevented by various interruptions to 
our work from giving a result which could lay claim to the greatest accuracy. In 
1878 we made at Pitlochry, in Perthshire, between 600 and 700 observations, but 
the toothed wheel which was made for us not having the number of teeth in it which 
we had ordered, we were not able to eliminate perfectly certain unknown quantities 
occurring in the formulae, and we felt that it would be better to wait until we could 
give a result in which we had perfect confidence. At the same time we resolved so 
to alter our apparatus that we should not have to depend upon the mean of a very 
large number of experiments to give us a good result, but that each observation should 
give us an accurate measurement, free from all doubt, This has now been accom- 
