CHAP, in.] SIGHT. 1245 



owing to the thinness or absence of all retinal layers except the 

 cones and cone fibres, still in ordinary vision we make large use of 

 the whole yellow spot, and our sensations of the colour of external 

 objects must be to a certain extent influenced by the pigment of 

 the spot. The light which reaches the rods and cones of this region 

 from objects which we call white, is in reality more or less tinged 

 with yellow; in other words what we call white is more or less 

 yellow. Indeed variations in the amount of pigment present in 

 the yellow spot have been offered in explanation of some of the 

 differences in colour vision discussed above ; but the explanation 

 does not seem satisfactory, since there is not such a difference 

 between the sensations derived through the yellow spot and those 

 derived through the colourless retina immediately around the 

 yellow spot as is required for the explanation ; in fact the presence 

 of the yellow pigment seems to affect our vision much less than 

 might be imagined. 



769. In speaking of the relation between a visual sensation 

 and the intensity of the stimulus ( 747) we were confining our 

 remarks to white light; when we inquire into the behaviour of 

 our colour sensations under variations in the intensity of the 

 stimulus, we come upon results which are in many ways com- 

 plicated. We must be content with pointing out one or two only 

 of these. 



Each of our colour sensations, when the light giving rise to it 

 reaches a certain intensity, ceases to be a colour sensation and 

 becomes a sensation of white. The theory of three primary colour 

 sensations may be used to explain this. Thus, taking violet as 

 a primary sensation, a violet light of moderate intensity appears 

 violet because it excites the primary sensation of violet much 

 more than those of green and red. If the stimulus be increased 

 the maximum of violet stimulation will be reached, while the 

 stimulation of green will continue to be increased and even that 

 of red to a slight degree. The result will be that the light appears 

 violet mixed with green, that is to say, appears blue. If the stimu- 

 lus be still further increased while the green and violet are both 

 still largely excited the red stimulation may be increased until 

 the result is violet, green, and red in the proportions which make 

 white light. And so with light of other colours. But the same 

 facts may also be explained on Hering's theory, for this supposes 

 that the stock, so to speak, of white-black substance is far greater 

 than that of either of the other two visual substances; hence 

 under violent stimulation the white sensation wholly overpowers 

 any accompanying colour sensation. 



Conversely when the intensity of the stimulus is diminished, 

 colour sensations may disappear before all sensation of light is 

 lost. When the light is very dim we cease to recognize the 

 colour of coloured objects though we continue to see the objects. 

 And this is not merely because the white light reflected from the 



