CHAP. HI.] , SIGHT. 1225 



sense that the red and yellow rays can unite together to form rays 

 of the same wave-lengths as the orange rays ; the three things are 

 absolutely different. It is simply the mixed sensation of the red 

 and yellow which is indistinguishable from the sensation of orange ; 

 the mixture is entirely and absolutely a subjective one. In the 

 same way we may by appropriate mixtures produce the sensations 

 corresponding to other parts of the spectrum. Now we must 

 suppose that rays of different wave-lengths affect the retina in 

 different ways and so give rise to different visual impulses, 

 that, for instance, the visual impulses generated by orange rays 

 are different from those generated by red rays or by yellow 

 rays. Hence we are led by the fact of mixed sensations being 

 identical with other apparently simple sensations to infer that 

 the visual impulses and hence the visual sensations which any 

 ray originates are of a complex character. We conclude, for 

 instance, that the impulses which a ray in the middle of the 

 orange gives rise to are not simple impulses answering exclusively 

 to the colour of that ray, but complex impulses, parts of which 

 may be excited by rays other than the particular orange ray in 

 question. In saying this we must bear in mind that we possess 

 no direct information of the nature of visual impulses, our know- 

 ledge of these being limited to what we learn through the 

 sensations to which they give rise ; the complexity of the sensation, 

 may be, and indeed probably is, of a different order from that of 

 the visual impulse ; to this point we shall return. 



The view that our ordinary colour sensations are mixtures of 

 simpler sensations is further confirmed by an examination of 

 the colours of external nature. For, though we see around us 

 very many colours besides those present in the spectrum, yet we 

 find that the sensations of all these colours may be reproduced 

 by mixtures of sensations excited by various parts of the 

 spectrum. Thus the colour purple, which is so abundant in the 

 external world and yet so conspicuous by its absence from the 

 spectrum, may be at once reproduced by fusing in proper pro- 

 portions the sensations of red and of blue. And very many 

 other colours present in the external world but not seen in the 

 spectrum itself may be produced by mixing various spectral 

 colours in various proportions. 



Other colours in nature may be reproduced by mixing spectral 

 colours with white or with black. When by means of a slit we 

 allow a certain limited part of the spectrum, say in the green, to 

 fall on a certain area of the retina, the rays exciting that area 

 have certain wave-lengths, lying within certain limits, say from 

 X 525 to X 535 ; no rays but these are affecting the retina at the 

 time, and the result is the sensation which we call spectral green. 

 But we might easily so arrange matters that a certain amount of 

 white light, that is of light of all wave-lengths of the visible 

 spectrum, should fall on the area in question at the same time 



