914 COLOUR VISION. [BOOK in. 



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 stimulus 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 pro- 

 portions 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 object (and it is through this that we chiefly become aware 

 of the form of an object) is more powerful than the particular 

 rays which give the object colour; since even a saturated colour 

 behaves in the same way. If with a feeble illumination we allow 

 a very small part of the spectrum to fall on the retina, we are 

 much more distinctly conscious of a sensation of light than of 

 any particular colour sensation ; indeed the minimum sensation 

 thus felt has been called a 'grey' for all parts of the spectrum. 

 Moreover the colour which is first recognized upon gradually 

 increasing the illumination, appears less saturated, that is to say 

 apparently more mixed with white than when a large amount of 

 light of the same refrangibility falls on the retina ; and such 

 distinct colour sensation as may be felt at the first moment of 

 looking at such a light soon diminishes, giving way to a mere 

 sensation of light. 



When we attempt to compare one colour sensation with 

 another in reference to their behaviour towards variations in the 

 intensity of the stimulus we find the results to a certain extent 

 conflicting. When we diminish the intensity of the stimulus by 

 diminishing general illumination, when we look for instance at 

 objects in nature under light of varing intensity, we find that 

 the colours change unequally as the light diminishes ; as is well 

 known the colours of flowers look very different when night is 



