VELOCITY OF WHITE AND OF COLOURED LIGHT. 
287 
Let us now consider some of the effects which must follow from these results. 
Refraction and dispersion. —There is nothing in the undulatory theory of light, 
independent of our views as to the kind of force acting in the ether, which is opposed 
to the view that red and blue lights travel with different velocities in vacuo. We 
know that the ratios of the velocities of red or blue lights, in a refracting medium 
and in vacuo, are equal to their refractive indices. But the theory of refraction or 
dispersion tells us nothing about the ratio of the velocities of red and blue lights 
either in vacuo or in refracting media. 
Interference and diffraction. —So with the phenomena of interference and diffrac¬ 
tion, which give us a measure of the wave-length of the different colours. If we could 
measure the period of vibration of the different colours, this would show us the 
difference in velocity in the different colours. But our knowledge of the period of 
vibration is dependent on the velocity of light, and these phenomena give us no 
information upon the phenomena we are discussing. 
Experiments of Cornu and Michelson. —'Why did not these experimenters notice 
the effect of colour 1 ? First, consider the work of Cornu. We might expect that he 
should have seen colours in his star of light near the eclipses. But when we have a 
very feeble light changing its intensity it is very difficult to appreciate differences in 
colour, and he had no light for comparison as we had. He depended largely for the 
accuracy of his results on two facts : (1) that he took a mean of 546 pairs of observa¬ 
tions, so reducing the probable error to one twenty-third part of that of a single pair 
of observations, and (2) that he used eclipses as high as the 21st eclipse, which reduces 
the personal error of the eye observation to one forty-first part. We may add that 
he had five phases (which he calls U, u, V, v, and v) differing slightly, and he had 
considerable latitude in placing any single observation in one or other of these phases. 
We may also add that into each set of observations he introduced what he called 
“ rectifications probables/ 7 which assisted to eliminate discordances. 
If, however, we examine his non-rectified results, and classify them according to the 
source of light used, we shall, we believe, obtain confirmation of our views. 
Neglecting the small correction depending upon the phase (U -\-u or Y -f- v) we 
have the following results:— 
Source of light. 
Number of 
observations. 
Approximate 
velocity. 
Kiloms. 
Petroleum .... 
20 
298,776 
Sun near horizon . 
77 
300,242 
Lime light .... 
449 
300,290 
Compare this with the absolute determinations given on p. 270. 
