278 Animal Life and Intelligence. 



" 3. Green excites strongly the fibres sensitive to green, 

 feebly the other two. 



"4. Blue excites moderately the fibres sensitive to 

 green and violet, feebly the red. 



" 5. Violet excites strongly the fibres sensitive to violet, 

 feebly the other two. 



" 6. When the excitation is nearly equal for the three 

 kinds of fibres, the sensation is white/' 



This theory cannot be regarded as more than a pro- 

 visional hypothesis. Still, by its means we can explain 

 many colour-phenomena. It is well known, for example, 

 that if we gaze steadily at a red object, and then look aside 

 at a grey surface, an after-image of the object will be seen 

 of a blue colour. According to the theory, the red fibres 

 have been tired and cannot so readily answer to stimulation. 

 Over this part of the retina, therefore, the effect of grey 

 light is to stimulate normally the fibres sensitive to green 

 and violet, but only slightly those sensitive to red, owing 

 to their tired condition. The result will be, as we see from 

 the above scheme (4), the sensation of blue. Colour-blind 

 people, on this view, are those in whom one set of the fibres, 

 generally the red or the green, are lacking or ill developed. 



We may, perhaps, with advantage restate this theory in 

 terms of chemical change, or metabolism. On this view 

 three kinds of "explosives" are developed in the retinal 

 cones ; for it is seemingly the cones, rather than the rods, 

 which are concerned in colour-vision. All three explosive 

 substances are unstable ; but one, which we may call E., 

 is especially unstable for the longer waves of the spectrum ; 

 another, G., for the waves of mid-period ; a third, V., for 

 those of smallest wave-length. 



Suppose that E. only were developed. If, then, we were 

 to look at a band of light spread out in spectrum wave- 

 lengths, we should see a band'* of monochromatic r. light. 

 Its centre would be bright, and here would be the maximum 

 instability of E. On either side it would fade away. The 



* A band and not a line, because K. is unstable to the impact of a con- 

 iderable range of light-vibratious. 



