584 



PHYSIOLOGY 



of cone, each being so made as to respond to one of the three fundamental colours, 

 namely red, green and violet. The impulses from these cones are so combined in the 

 brain that they give a complete picture of the separate coloured images. When all 

 three types of cone are equally stimulated, a colourless sensation results. Each visual 

 unit may therefore be regarded as consisting of three cones, one of which responds 

 to each of the fundamental colours. From this we should expect that the limit to the 

 acuteness of vision would be reached when the separation of ' the images on the retina 

 is not less than the diameter of such a unit. But the diameter of the foveal cones is 

 approximately 0-C025 mm., and therefore that of a unit would be roughly 0-004 mm. 

 Now it is found by experiment that the limit to the acuteness of vision is reached when 

 the retinal images are separated by about 0-002. It is therefore clear that the unit 

 cannot be larger than one cone, and that in consequence each cone must be capable 

 of responding to all three fundamental colours. In consequence of this Helmholtz 

 made the suggestion that there are three different chemical substances, each of which 

 undergoes alteration under the influence of one of the three fundamental colours. 

 The breakdown products thus formed stimulate the cones in proportion to the amount 

 in which they are present, their function in this respect being comparable to the taste 

 buds of the tongue. In this way each cone can respond to all three colours and also 

 to white light, and therefore the requirements of visual acuity are satisfied. As to what 

 these chemical substances are, we at present know nothing ; it has been suggested that 

 the substance responsible for the perception of blue is a pigment discovered by Ktihne 

 called visual yellow, and visual purple might from its absorption curve provide the 

 pigment for the green, but at present we have no evidence for this. It should be noted 

 that the three sensations are brighter and more saturated than the three fundamental 

 colours with which they may be said to correspond. This follows from the researches 

 of Maxwell and Abney, which showed that each of the fundamental colours stimulates 

 to some extent the other sensations beside its own. Thus blue light stimulates the 

 green and red sensations to a certain extent, green similarly the red and blue sensations, 

 but red the green slightly, and the blue not at all. This view as to the greater saturation 

 of the sensations finds some confirmatory evidence from the increase which the satura- 

 tion of a colour undergoes after the eye has been stimulated by its complementary. 



v - V G & v 



too 



90 

 80 

 70 

 60 

 50 

 40 

 30 

 20 

 10 





t 



ON 





\ 



\y 



X 



PAVE SENSATION 



64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28 26 24 



' ' ' 



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6o|oo ' 



i 



FIG. 299. The red, green and hlu> sensation curves and the luminosity curve of 

 white light. Luminosity vertical wavelengths horizontal. 



Having thus briefly outlined the hypothesis of Young and Helmholtz, the n-lnti.m 

 ship with the results of experiment may receive consideration. 



