66 MR. J. SMITH ON THE ORIGIN OF COLOUR 



Circular Figures. 



In order if possible to simplify this problem, I tried 

 discs with definite ratios of white and black, and by fixing 

 them at the centre I made them first revolve with 



Concentric Motion. 



99. These discs were made in this manner : I took a 

 piece of Bristol board, and having described on it several 

 concentric circles, L cut out with a knife, or painted with 

 lamp black, a portion of each of the rings formed by two 

 of these circles, say a half, a third, two thirds, or three 

 fourths, or any other proportions, as in plate IV. figs. 4, g, 

 6 and 7. I then fixed the disc on the machine, and made 

 it revolve in bright light. By this means the white and 

 black portions of the discs were made to pass alternately 

 over the same spot on the retina. The result was, that 

 when revolving neither the white nor the black parts were 

 seen, but the whole disc appeared beautifully coloured 

 with concentric rings, and each ring had its own shade of 

 colour; purples, greens and yellows predominating. 



When this method of experimenting is adopted each re- 

 volution of the machine may be considered as representing 

 the time or duration of a vibration, and the light reflected 

 from each ring, combined with the velocity, as the mo- 

 mentum of the ray. When the revolutions amount to 

 from six to eight in the second of time, colour is generally 

 formed so as to be seen by the majority of eyes. Some per- 

 sons can even perceive distinct colours when the revolutions 

 are twice or three times as numerous. At thirty-two revolu- 

 tions in the second of time, I see no colour, only the geo- 

 metrical figure generated by the motion and shaded grey, 

 like so many penumbrse, forming the ground, as it were, 

 on which the colours are produced when the light is sufii- 

 cient. 



