280 
DR. J. YOUNG AND PROFESSOR G. FORBES ON THE 
“ A blue, B red. Increase of speed increases brightness of B. [This is in accord¬ 
ance with previous experience.] This was with speed about 400. 
“ It seems then that to-night A is intrinsically bluer than B, owing probably to 
adjustment of focus. But will this explain all V’ 
Let us see now what would be the effect of A being intrinsically bluer than B. 
The following figure represents such a case on the supposition that blue and red light 
travel with the same velocity, and that A is deficient in red light. Full lines 
represent blue light, and dotted lines red. These coincide for B. 
Fig. 4. 
The result is clear that at different successive phases the blue and red rays 
alternately require a greater speed of rotation to produce equality of lights. But 
this does not in the least represent the results of our observations. 
The following diagram illustrates the case where A is deficient in red light, on the 
assumption that blue rays travel quickest. 
Fig. 5. 
The maxima of A and B with different speeds are indicated by the letters A, B. 
The red is shown to reach its maximum before the blue. The red in A is shown to 
have a smaller maximum than the red in B. The intersections of the full and dotted 
lines represent the speeds producing equality of the blue and red rays respectively. 
Here Sy is almost nil, whereas is too great. Hence, if we observe only at one 
speed, we may get far too great or far too small a difference between the velocities of 
red and blue light. 
The mean of Sy and Sx is (nearly) that due to the difference in velocity of red and 
blue rays. 
[N.B ,—Our observations on February 28 were taken at the 12th and at the 13th 
equalities, so that this cause of uncertainty is eliminated.] 
