COLOUR VISION. 175 



responsiveness to a stimulus which tends to bring about the oppo- 

 site reflex becomes increased. Fatigue following a reaction of one 

 kind makes the way easier for the opposing reaction. 



In more detailed form, according to the Young theory, the reason why the 

 after-image of a coloured object is tinged with the complementary colour is as 

 follows. During the time that the gaze was fixed on the object all three com- 

 ponents in the cells of the retina on which the image fell were being stimulated but 

 the component or components which are most sensitive to the colour of the object 

 were being stimulated most. When the object is withdrawn and the place on 

 which the image falls is uniformly stimulated by white light all three com- 

 ponents respond. The one which was mainly affected by the former light is 

 fatigued however and responds less than the others. This enables the colour 

 complementary to it to come into prominence. 



PARTIAL COLOUR BLINDNESS. This abnormal condition of vision is inter- 

 esting in the discussion of the theories of colour vision as it affords a comparison 

 with the normal. The defect, fairly common among men, less so in women, 

 is physiologically quite different from total colour blindness, with which it must 

 not be confused. People who are partially colour blind distinguish differences 

 in light waves of different lengths, no matter whether their intensities are equal 

 or not, but they see fewer differences than normal people. If they are shown 

 light of different colours (the best test is light coming from lanterns with windows 

 of coloured glass) they select as being of similar shades the yellows and the blues 

 and the violets which look much alike to the normal eye, but in choosing shades 

 similar to a green they also take, as well as other green lights, different shades of 

 red, and in matching a red standard they pick out as having a colour similar to it 

 not only other reds but also green as well. The condition of their light- receptive 

 mechanism seems to be not so much different from, as simpler than, the normal; 

 roughly speaking, partially colour-blind vision is a reduction form of ordinary 

 sight. Whereas all the various shades which one sees with normal colour vision 

 cannot be produced with combinations of less than three given colours, two only, 

 if mixed in different proportions, will match all the colours seen by a subject who 

 is partially colour-blind. 



There are two types of the defect. In the first, most common, form the eye 

 is relatively insensitive to red light. To match an olive green, people with this 

 type of the defect choose a scarlet which to the ordinary eye appears much brighter 

 and the visible spectrum is shorter for them at the red end than it is for other 

 people. Subjects whose vision belongs to the second type match a green with a 

 red which is about equally bright and the red end of the spectrum appears to have 

 the usual length. 



The explanation of these facts, according to the theory of the three visual 

 components, is that in eyes of the first type the red component is left out and 

 that all wave lengths have only the other two components to act upon. In the 

 second type it is the green which is missing. This would express the actual con- 

 ditions well enough if only these abnormal types were accurately, and not merely 

 approximately, reduction type of the normal. More recent work however has 



