THE SENSATION OF SIGHT. 241 



with yellowish green and violet would appear very bright, 

 although both surfaces alike seem to the eye to be simply 

 white. Again, if we successively illuminate coloured ob- 

 jects with white beams of light of various composition, 

 they will appear differently coloured. And whenever we 

 decompose two such beams by a prism, or look at them 

 through a coloured glass, the difference between them at 

 once becomes evident. 



Other colours, also, especially when they are not 

 strongly pronounced, may, like pure white light, be 

 composed of very different mixtures, and yet appear in- 

 distinguishable to the eye, while in every other property, 

 physical or chemical, they are entirely distinct. 



Newton first showed how to represent the system of 

 colours distinguishable to the eye in a simple diagram- 

 matic form ; and by the same means it is comparatively 

 easy to demonstrate the law of the combination of colours. 

 The primary colours of the spectrum are arranged in a 

 series around the circumference of a circle, beginning with 

 red, and by imperceptible degrees passing through the 

 various hues of the rainbow to violet. The red and violet 

 are united by shades of purple, which on the one side pass 

 off to the indigo and blue tints, and on the other through 

 crimson and scarlet to orange. The middle of the circle 

 is left white, and on lines which run from the centre to 

 the circumference are represented the various tints which 

 can be produced by diluting the full colours of the cir- 

 cumference until they pass into white. A colour-disc of 

 this kind shows all the varieties of hue which can be 

 produced with the same amount of light. 



It will now be found possible so to arrange the places 

 of the several colours in this diagram, and the quantity 

 of light which each reflects, that when we have ascer- 

 tained the resultants of two colours of different known 



