CHAP. IIL] SIGHT. 1229 



spectrum. If we take, for instance, a red of a certain wave-length, 

 a green of a certain wave-length, and a blue of a certain wave- 

 length, we can, without calling to our aid any other wave-lengths, 

 by varying the relative intensities of the three, produce not only 

 white light, but also orange, yellow, and violet, with all the 

 intermediate tints, that is to say, produce all the colours of the 

 spectrum ; and we may in the same way produce the non-spectral 

 purple. Our choice however is to a certain extent limited ; the 

 three colours which we choose must be spread over the spectrum, 

 for we cannot obtain these results with three colours taken from 

 the red and yellow alone, or from the green and blue alone. 

 Moreover, the result is not a complete one ; the colour which we 

 thus produce by combining three spectral colours differs from a 

 true spectral colour in not being saturated ; it is " mixed with 

 white," more so in some cases than in others ; in relation to this 

 deficiency of saturation, the green region of the spectrum behaves 

 differently from the red end and the blue end. 



761. These results shew that the primary colour sensations 

 out of which our recognized colour sensations originate, may be 

 reduced to three in number. If we suppose that we possess three 

 primary sensations so disposed in reference to the spectrum, so 

 arranged so to speak along the spectrum } that a ray of light affects 

 each of the three differently according to its wave-length, we can 

 understand how all our multitudinous colour sensations may arise 

 from the varied excitation of these primary sensations. There 

 may be more than three of these primary sensations, but if so they 

 must behave as if they were three ; they cannot be less, since as 

 we have seen the results of mixing two sensations only are 

 extremely limited. We may therefore speak of our vision as 

 trichromic, as based on three, or the equivalent of three, primary 

 sensations. 



When we attempt to inquire further into the nature of these 

 primary sensations, we find ourselves in the face of two rival 

 theories. 



The one, propounded by Young but more fully elaborated 

 by Helmholtz and Maxwell, and known as the Young-Helmholtz 

 theory, teaches that there are three and only three such primary 

 sensations. As we have just seen, any three parts of the 

 spectrum, with certain restrictions, might be chosen as corre- 

 sponding to these three primary sensations so far as concerns 

 the reproduction, by means of them, of all other colour sensations ; 

 hence in determining the nature of the primary sensations we 

 must have recourse to other considerations. We may for in- 

 stance very naturally suppose that two of the three correspond 

 to the two ends of the spectrum, and may therefore be spoken 

 of as more or less closely corresponding to our recognized 

 sensations of red, and of violet. If red and violet be thus 

 two of the sensations the third one must correspond to green, 



