VISION 385 



wavy, showing that their images are taken up, not by a con- 

 tinuous substance, but by the mosaic of cones. So far the 

 explanation of the visual unit is strictly anatomical ; but it 

 must be added that trained observers can recognize the 

 separateness of objects which subtend angles of much less 

 than 60 seconds not more than 5 or 6 seconds. This can be 

 accounted for only on the hypothesis that images far closer 

 together than the width of a cone produce a specific effect in 

 passing across the anatomical unit. 



In 1807 Thomas Young, the physicist, formulated a theory 

 to account for colour- vision. He supposed that the retina 

 contains three kinds of apparatus a, b, and c each especially 

 responsive to a particular kind of light, all three slightly 

 stimulated by rays of all colours. (Young imagined three 

 kinds of nerve, but modern supporters of his theory suppose 

 three different substances chemically changed by light.) A 

 prism spreads out the rays which are combined in white light 

 into a band in the order of their wave-lengths those which 

 have the longest wave-length (08 /^) and the slowest rate of 

 vibration (381 billions to the second) at one end, those which 

 have the shortest wave-length (0-4 //-) and the most rapid 

 vibration (764 billions to the second) at the other : between 

 these two extremes every intermediate grade of length and 

 rapidity. These are a mere fraction a small group of the 

 waves which the aether transmits, but they are all that we 

 can see. The long, slow vibrations give rise to sensations 

 which we describe as red ; the short, rapid vibrations we 

 describe as violet. Our names for the tints which intervene 

 are singularly old-fashioned and unsatisfactory, but all persons 

 agree that they recognize in the spectrum a certain number of 

 definite colours. Some normal-sighted persons say twelve, 

 others eighteen. It is largely a question of terminology. 



Many considerations show that it is quite unnecessary to 

 imagine that the retina is affected in a different kind of 

 way by every kind of light, or by each j.of several groups 

 of waves. If the red of the spectrum is mixed with yellow, 

 we receive an impression of orange, which is identical with the 

 impression produced by waves of the mean length of red and 

 yellow ; orange and green give yellow ; yellow and blue, green. 

 Any two complementary colours yield white. By taking three 



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