J ' 4 DR. J. YOUNG AND PROFESSOR G. FORBES ON THE 
increasing with respect to the other, with increase of velocity [of the toothed wheel], 
appears red ; and the other one blue.” The words in square brackets are not in the 
original, and are inserted here to make the statement clearer. 
These observations clearly proved to us that the colour which we had often observed 
was not always due to the adjustment of the distant reflectors. For here sometimes 
the one and sometimes the other was the red one. At each successive equality 
( e -9’> li ie 11th and 12th, the 12th and 13th, &c.) the colours of A and B are reversed. 
Since February 11 there certainly have been many days when the colour-differences 
were not perceptible. It may perhaps have been because the stars were not steady or 
were flickering or indistinct. On these occasions the atmospheric refraction disturbs 
the course of the rays, so that the teeth of the wheel being extremely minute, a ray of 
light which, if there were no irregular atmospheric refraction, would not reach the 
reflector, does so under these circumstances. In such a case the stars do not alter 
their intensities, with change of speed of the toothed wheel, so regularly as they do 
when the atmosphere is not unequally heated and disturbed. 
The general result however was established by the observations on February 11, 
1881, but it is not a common observation. 
Explanation of the colours perceived in the return light. 
The simplest explanation which can be given of these phenomena, and the only 
explanation which seems to be capable of standing all kinds of tests, is that the 
different colours travel with different, velocities, the more refrangible rays , or those with 
shortest wave-length , travelling quickest. If this were the case we should be forced 
to alter our diagram indicating the intensities of A and B. We should have as many 
curves of intensity for each of the two stars of light as there are colours in the light 
we are employing. Let us examine only two of these colours (red and blue for 
example). If the red light travel slower than the blue a smaller velocity is required 
to produce an eclipse with red light and with blue. For this reason the curve repre¬ 
senting intensity in terms of speed of rotation for red light will have its maxima and 
minima lagging gradually more and more behind those for the blue light. We are in 
general dealing only with the speeds of rotation which produce the 12th, 13th, and 
14th equalities ; and during that small variation in speed the lines for red and for blue 
light may, for purposes of illustration, be drawn sensibly parallel. The curves for the 
two stars A and B would then be shown approximately by the following diagram, in 
which dotted lines represent red light, and full lines blue light. Here we notice that 
at x the light of A is diminishing with increase of speed, and the abscissa correspond¬ 
ing to blue light is greater than that corresponding to red light. Hence, ivhen the 
intensity is diminishing with increase of speed the star should have a blue tinge. But 
at y the light of A is increasing with increase of speed, and the abscissa corresponding 
to red light is greater than that corresponding to blue light. Hence, when the 
