COLOUR-VISION 435 



Thus, looking at a bright light rapidly renders the part o!' 

 the retina on which the liyht fall^, insensible ; and on 

 looking from the bright light towards a moderately-lighted 

 surface, a dark spot, arising from a temporary blindness 

 of the retina in this part, appears in the field of view. If 

 the bright light be of one colour, the part of the retina on 

 which it falls becomes insensible to the rays of that colour, 

 but not to the other rays of the spectrum. This is the 

 explanation of the appearance of what are called after- 

 images. For example, if, as in the form in which the 

 experiment is most commonly made, a bright red wafer be 

 stuck upon a sheet of white paper, and steadily looked at 

 for some time with one eye, when the eye is turned aside to 

 the white paper a greenish spot will appear, of about the 

 size and shape of the wafer. The red image has, in fact, 

 fatigued the part of the retina on which it fell for red 

 light, but has left it sensitive to the remaining coloured 

 rays of which white light is composed. But we know that 

 if from the variously coloured rays which make up the 

 spectrum of white light we take away all the red rays, the 

 remaining rays together make up a sort of green. So that, 

 when white light falls upon this part, the red rays in the 

 white light having no effect, the result of the operation of 

 the others is a greenish hue. The colour of the after-image 

 is thus of necessity complementary to that of the object 

 looked at. If the wafer be green, the after-image is of 

 course red. 



Colour-blindness.— Most people agree very closely 

 as to differences between different colours and different 

 parts of the spectrum. But there are exceptions. Thus 

 a certain number of persons see very little difference 

 between the colour which most people call red, and that 

 which most people call green. Such colour-blind persons 

 are unable to distinguish between the leaves of a 

 cherry-tree and its fruit by the colour of the two ; 

 they are only aware of a difference of shape between 

 the two. Cases of this "red-blindness" or "red- 



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