212 RECENT PROGRESS OF THE THEORY OF VISION. 



is reflected from it, and the two combined pass on to the retina 

 at o. In general, then, light, which consists of undulations of 

 different wave-lengths, produces different impressions upon our 

 eye, namely, those of different colours. But the number of 

 hues which we can recognise is much smaller than that of the 

 various possible combinations of rays with different wave-lengths 

 which external objects can convey to our eyes. The retina 

 cannot distinguish between the white which is produced by the 

 union of scarlet and bluish-green light, and that which is com- 

 posed of yellowish-green and violet, or of yellow and ultramarine 

 blue, or of red, green, and violet, or of all the colours of the 

 spectrum united. All these combinations appear identically as 

 white ; and yet, from a physical point of view, they are very 

 different. In fact, the only resemblance between the several 

 combinations just mentioned is, that they are indistinguishable 

 to the human eye. For instance, a surface illuminated with 

 red and bluish-green light would come out black in a photograph ; 

 while another lighted 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 objects with white beams of light of various composi- 

 tion, 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 batween 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 indistinguishable 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 diagrammatic 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 



