EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 147 



half of the disc might be seen by transmitted light blue, and the other by 

 reflected light white. In the course of the reflected light I placed a yellow 

 glass, and in this way I had two nearly coincident images of the slits, one 

 yellow and one blue. By turning the disc slowly, I observed that in some 

 parts the yellow slits and the blue slits appeared to pass over the field alter- 

 nately, while in others they appeared superimposed, so as to produce alternately 

 tfoeir mixture, which was pale pink, and complete darkness. As long as the 

 disc moved slowly I could perceive this, but when the speed became great, the 

 whole field appeared uniformly coloured pink, so that those parts in which the 

 colours were seen successively were indistinguishable from those in which they 

 were presented together to the eye. 



Another form in which the experiment may be tried requires only the 

 colour-top above described. The disc should be covered with alternate sectors 

 of any two colours, say red and green, disposed alternately in four quadrants. 

 By placing a piece of glass above the top, in the plane of the axis, we make 

 the image of one half seen by reflection coincide with that of the other seen 

 by transmission. It will then be seen that, in the diameters of the top which 

 are parallel and perpendicular to the plane of reflection, the transmitted green 

 coincides with the reflected green, and the transmitted red with the reflected 

 red, so that the result is always either pure red or pure green. But in the 

 diameters intermediate to these, the transmitted red coincides with the reflected 

 green, and vice versa, so that the pure colours are never seen, but only their 

 mixtures. As long as the top is spun slowly, these parts of the disc will 

 appear more steady in colour than those in which the greatest alternations 

 take place ; but when the speed is sufficiently increased, the disc appears per- 

 fectly uniform in colour. From these experiments it appears, that the apparent 

 mixture of colours is not due to a mechanical superposition of vibrations, or 

 to any mutual action of the mixed rays, but to some cause residing in the 

 constitution of the apparatus of vision. 



(7) Presentation of the Colours to be mixed one to each Eye. 



This method is said not to succeed with some people ; but I have always 

 found that the mixture of colours was perfect, although it was difficult to con- 

 ceive the objects seen by the two eyes as identical. In using the spectacles, 



192 



