272 
DR. J. YOUNG AND PROFESSOR G. FORBES ON THE 
brighter, while B is still diminishing in brightness, and at a certain speed they are of 
equal brightness. In the same way in the different phases, as the speed of the wheel 
is increased more and more, we have a succession of equalities in the two stars. The 
distance of A from the observer is to that of B in the ratio of about 13 to 12. 
Hence it follows that the sixth maximum of B coincides with the seventh minimum 
of A. In the following figure abscissae represent speeds of revolution of the toothed 
wheel, and ordinates intensities of the stars. The intersection of the two lines indi¬ 
cates equality of brightness and shows the speed required to produce it. 
Fig. 2. 
The lines in the above figure which indicate the brightness are subject to certain 
alterations dependant upon the necessary imperfections in the optical and mechanical 
parts of the apparatus. It has been shown in the previous part of this memoir that 
our method is in general unaffected by these alterations. Most of our observations 
have been made at speeds corresponding to the 12th, 13th, and 14th equalities, and it 
will be unnecessary in the diagrams which represent further development of the 
theory to delineate other parts of the diagram. 
Distinctive colours observed in the return light. 
Having made these preliminary remarks, we will now proceed to trace the steps by 
means of which the relation between colour and velocity has been suspected, and the 
quantity of the effect has been approximately determined. 
In the course of our observations made with sunlight at Pitlochry in 1878, and in 
those made with the electric light at Kelly in 1880-81, we were frequently annoyed 
by the presence of colour in the stars, one of them appearing reddish and the other 
bluish. This made it very difficult to appreciate the exact speed which might be said 
to produce equality in the lights; for, as is well known, it is very difficult, if not 
impossible, to judge accurately of the equality of two lights of different colours. We 
considered that these colours arose from a want of accurate adjustment of the distant 
reflectors. These consist each of a telescope tube with an achromatic object-glass at 
one end, but with no eye-piece, and having a silver mirror at the focus of the object- 
glass. An image of the object-glass of the observing telescope is thrown by the rays 
from the source of light upon the silver mirror, whence the light is reflected back to 
the observing telescope. Now the quantity of light which is reflected back into the 
observing telescope depends largely upon the accuracy of focus of the reflecting colli- 
