73 MR. J. SMITH ON THE ORIGIN OF COLOUR 



of a prism, one reflecting liglit, the other no-light to the 

 spectator. On this disc, when one ring was painted black 

 on the left side of the centre, the next was painted black 

 on the right side, and so on alternately. Hence when in 

 rotation, as the white of any ring diminished the black 

 increased, or vice versa as the white increased the black 

 diminished, and hence the light on the disc appeared as it 

 were stratified, and to come from two different directions, 

 or to be in two different planes. 



Now this disc {plate IV. fig. 9) when fixed on the machine 

 at the centre, 0, produced precisely the same effect as fig. 8 

 when fixed out of the centre, as at e or e, that is at exactly 

 half the breadth of one of the rings from the centre 0. In 

 both the contiguous rings were of different colours — one 

 being of a yellowish or reddish tint, the next of a purple 

 or greenish tint; and the alternate rings had the same 

 colour. On reversing the motion, in both figures the rings 

 which were yellow became purple, and vice versa the 

 purple became yellow. 



110. It will be remarked as a part of this very singular 

 phenomenon, that when the light increases in relation to 

 the black or shadow, it appears redder than when it dimi- 

 nishes. In the latter case it appears green or blue, or 

 purple. 



In order to analyse the phenomena exhibited by this 

 disc still further, I formed two similar semi-discs, and 

 drew on them seven concentric rings, each of which was 

 divided into two equal parts, as before. But instead of 

 painting each alternate ring on opposite sides of the disc, 

 I painted one disc only on the right of the centre, the 

 other only on the left. The former disc was thus divided 

 into two parts, one representing the right side, the other 

 the left, as in plate V.figs. 1 and 2. I now expected that, 

 the colours which were only repeated alternately on the 

 former disc, would be repeated continuously on these; 



