68 MR. J. SMITH ON THE ORIGIN OF COLOUIl 



already had^ it would virtually be making one disc revolve 

 over another. I therefore cut out a portion — a quarter, 

 or a half — of a ring from the centre of each disc, so that 

 the centre of motion might be made to slide along the 

 diameter at pleasure. By this process the white of one 

 ring Avould gradually pass on to the black of another, and 

 vice versa. In this way I added greatly to the variety of 

 colour, and, as will be seen on examination, doubled the 

 number of rings which were originally described on each 

 disc. Thus the portion a a, plate IV. figs. 4, 5, &c., was 

 cut out, and the centre of motion was fixed at pleasure on 

 any part of the line, or even of the curve. 



An examination of the figure whilst in motion will show 

 what takes place. The effect is the same as if there were 

 two discs in motion, one passing over the other; one 

 having for a radius the larger portion of the original 

 diameter, the radius of the other being the smaller por- 

 tion of the same. 



Spiral Figures. 



102. The experiment just described having proved very 

 successful, my next device was to employ a sort of spiral 

 figure of white on a black ground. I expected, from the 

 nature of the figure, to obtain very graduated tints. The 

 figures, however, which I at first used were so dark that, 

 although a great variety of colour was produced, the disc 

 appeared like a piece of printed calico, without that gra- 

 dation which I was in search of. Nevertheless I obtained 

 a very interesting set of colours. 



103. Although these colours were not exactly what I 

 was in search of, still the experiment was valuable, for 

 it showed that the plan I was pursuing would lead me to 

 the end I had in view. I therefore drew on the Bristol 

 board two or three concentric circles, cut the central one 

 and a great portion of the others awaj^, in order to give 



