xix THK KYI AM) Till. SKNSE OF SIGHT 219 



different sensations which we call colours. The retina may 

 similarly be fatigued for one colour only, that is, for one por- 

 tion only of white light. If, for instance, a bit of red paper 

 lying on a sheet of white paper be looked at steadily for some 

 time and then suddenly removed, a green patch will be seen 

 where the red patch lay. This may be explained by supposing 

 that the part of the retina on which the image of the red 

 patch fell became tired to red rays, and after the red patch 

 was removed, had no effect produced on it by the red rays 

 which were present in the white light now coming from the 

 same spot. It is as if the white light coming from the spot 

 contained no red rays ; but if you take the red rays out of 

 white light, the result is green, or to put it in another way, red 

 and green light mixed together make white light. Red and 

 green are said to be complementary colours ; when the eye is 

 fatigued for one the other is seen. The same is true of yellow 

 and blue. 



To some persons the differences between some colours do 

 not seem the same as they do to ordinary persons. They are 

 colour blind. It is found, for instance, that rather more 

 than one person in every hundred do not see red and green 

 as distinct colours, and cannot tell the difference between red 

 and green things except by their shade and brightness. This 

 is a very important matter in the selection of engine-drivers 

 and sailors. A few persons have been found to be totally 

 colour-blind, to whom everything appears of the same tint ; 

 but this is very ran-. 



The sensation of light may be excited in us by other means 

 than the falling of light on the retina. Firm pressure on the 

 eye causes a luminous image, and a blow on the eye, or a fall 

 on the head, may cause flashes of light. These effects prob- 

 ably have their origin in the retina itself. It is worthy of 

 notice that the sensations thus produced, though not caused by 

 li^ht, seem to us sensations of light; we "refer" them to 

 sources of lijht in the world around us. This should be 

 considered in connection with what was said a little while ago 

 about the inverted image on the retina. 



