Sight, and the Eye. 441 



formed by a color, or anything else, for that matter, is the same in one 

 person that it is in another. An individual having received a sensation 

 which he calls yellow, after the instruction of others, will compare suc- 

 ceeding sensations with the first, and name them yellow or otherwise, 

 as they appear to agree or not. 



I think, however, if the probable evolution of the cones be considered, 

 we shall find reason to modify Young's theory somewhat. First, we 

 must distinguish between a simple color and a compound color, which 

 may, with propriety, be designated, respectively, a true and a false 

 color. The first is the objective impression of waves of a particular 

 and uniform length, and is the color as expressed in the solar spectrum. 

 The second arises as the resultant of two sensations, one above and the 

 other below the true color. If an inch be divided into one hundred 

 million parts, the length of a wave of middle yellow light from the sun 

 would be represented, according to our table in chapter 40, by 2 172 

 parts. But the sensation which waves of this length gives, can be ap- 

 proximately counterfeited by the simultaneous action of the red and 

 green rays, respective!}', 2441 and 2016 parts long, the average of 

 these figures being 2228 a dark shade of }*ellow. Now, if we suppose 

 the cones to be differentiated from the rods by the action of the various 

 waves, it is conceivable that according to Young's theory we should in- 

 deed get the sensation of yellow from the joint action of red and green, 

 because there would be cones susceptible to red, and others susceptible 

 to green agitations ; but if a ray of pure solar yellow light should strike 

 these cones they ought not to be affected, and there being no cones dif- 

 ferentiated to the action of waves of 2172 parts, the eye would be blind 

 to this true yellow, while it could see the false or compound. If it be 

 said that the three kinds of cones can be agitated by the waves of dif- 

 ferent lengths, then why suppose three sorts of cones? Perhaps there 

 are onlv two or even only one. But the experiments showing fatigue 

 by looking at one color, while the eye is fresh for other colors, prove 

 the cones to be several kinds, and if that is so, each kind must have 

 been so developed by the action of one simple kind of light. We might 

 reasonably suppose that the colors most predominant in nature would 

 be the ones to make their mark on the plastic organism, such as the blue 

 of the sky and the green of the foliage, &c. The blue of the sky is 

 probably a simple color, being the refraction from fine particles of mat- 

 ter suspended in the air, or from the molecules of the air itself, of a di- 

 ameter equal to the length of blue waves. But, as we have seen, there 

 is reason to think that the green of chlorophyl is compounded of yellow 

 and blue, the green seen in foliage, or at least much of it, would tend 

 to develop in the eye not green seeing cones but yellow and blue ones. 

 It is said that five per cent, of Europeans and European- Americans are 



