42 Proceedings of the Royal Society 
for a short time, the eye is directed to the drawing, the wheel, or 
whatever it is, appears to he rotating in a direction the opposite of 
that of the exciting disc. Or we may vary the experiment, and 
look at a sheet of paper having a mottled surface.* When We do 
so, after looking at the rotating disc, we see a circular spectrum of 
the disc, in which the markings on the paper no longer appear 
stationary, but all seem moving in a circular direction, like a slow- 
moving whirlpool, the direction of the motion being the opposite of 
that of the exciting disc. 
In these cases the motion was circular, and the result was a circu- 
lar spectral motion. We may, however, vary the experiment, when 
we shall find that the spectrum will always correspond to the excit- 
ing motion. Suppose we take an endless band of paper, with black 
bars painted across it, and pass it over two drums revolving on 
horizontal shafts, one placed over the other, so that the paper band 
shall move in a straight line either upwards or downwards. If, 
while this band is in motion, the eye be fixed upon it for a short 
time and then the gaze be directed to the mottled paper, a spectrum 
of the moving band will be seen. A narrow strip of the mottlings 
will appear to flow through the mottled sheet of paper, reminding 
one strongly of the appearance of a lava stream, the breadth of the 
stream corresponding to the breadth of the moving paper band, and 
the direction of its apparent motion being the opposite of the mov- 
ing band first looked at. 
Or we may vary the experiment in this way: — Take a wheel, 
having spiral spokes coloured black, and rotate it in front of a disc 
similar to that first described, but in this case kept stationary, so 
that the white parts seem to travel from the centre to the circum- 
ference, or from the circumference to the centre of the disc, accord- 
ing to the direction in which the wheel is turned. If the mottled 
paper is looked at after looking at the apparatus in motion, all the 
mottlings in the spectrum seem to be in motion, either towards the 
* The markings on the paper should not be too strongly contrasted with the 
paper. White paper roughly spotted over with ink will do, but the effect is 
greatly increased when the contrast is not so great. The best effect is pro- 
duced by first washing the paper all over with Indian ink, thick enough to 
make the paper a darkish grey, and, while still wet, daubing it all over with 
Indian ink. In the absence of anything better, a cocoa-nut fibre mat does 
well. 
