A New Hypothesis concerning Vision. 27 o 



the law of maximal stimulation, which states that when a stimulus 

 is increased beyond a certain amount it is not followed by any in- 

 creased sensory effect. If you illuminate, say, a piece of red paper 

 with an intensely brilliant light, it appears white, for the red pig- 

 ment is unable to absorb all the blue and green spectral rays, which 

 it would be able to do by a medium illumination, and enough of 

 these green and blue rays are reflected to produce a maximum effect, 

 and the red and yellow rays, though no doubt falling in greater 

 quantity on the retina, produce likewise their full sensory effect and 

 no more. 



As the eye has been evolved by the action of the common 

 pigments of nature, their examination throws light upon some of 

 the facts of vision, and the sensory results of stimuli composed of 

 certain mixtures of spectral rays may be explained from the evolu- 

 tionary standpoint. If spectral rays near to each other, such as red 

 and green, be mixed, their colour is that of the spectral ray which 

 lies between them, in this case yellow. Now, when common natural 

 pigments are observed spectroscopically, they are seen to transmit 

 broad bands of spectral rays, generally extending to parts of the 

 spectrum other than that part which corresponds in colour to that 

 of the pigment. Thus a yellow natural pigment transmits a full 

 flood of red, yellow, and green spectral rays. If we put it another 

 way, the sensation yellow has in the course of evolution been produced 

 by pigments which stimulate the eye by yellow spectral ray plus 

 red and green spectral rays. These red and green spectral rays given 

 out by natural pigments do not give rise to their respective sensations 

 when mixed, for there is no such thing as a red-green sensation ; but 

 they intensify the yellow sensation which would be produced to a 

 less extent by the intermediate yellow ray when acting alone. It is 

 a fact, beyond which we cannot go, that the combination red plus 

 green spectral ray stimulating the eye whenever we regard a yellow 

 pigment produces the sensation we call yellow ; an artificial mixture 

 of such spectral rays of course gives rise to the same sensation. 

 Similar explanations hold for the mixtures of green and violet, &c. 



It is a fact that a sensation of white or grey is produced (a) when 

 the eye is stimulated by all the spectral rays, (6) when it is stimu- 

 lated, as shown by Helmholtz, by certain pairs, e.g., red and blue- 

 green. It is suggested that the colour top of Maxwell has, as a 

 physiological experiment, been misinterpreted. When you mix on 

 the disc a blue and yellow and get grey, the bine paper transmits 

 to the eye one-half of the spectrum, viz., violet, blue, and some 

 green, and the yellow paper transmits the other half, viz., some 

 green, yellow, and red. You are therefore looking at what is physically 

 the same stimulus as that given by a piece of white paper seen in half 

 light. That the sensation grey occurs is not to be wondered at, for 



