COLOUR VISION AND COLOUR-VISION THEORIES 483 



The visual acuity corresponds roughly to the distribution of 

 the cones. Though the rods are much more numerous in the 

 periphery of the retina, visual acuity is very much less with 

 this part. 



The Relation behveen the Foveal and the Para-foveal Regions. — 

 As there are no rods in the fovea, if the rods and cones were 

 percipient elements of a different character, there ought to be a 

 qualitative difference between these regions. It has, however, 

 been conclusively proved that there are only gradual quantita- 

 tive differences in the sight between the foveal and the para- 

 foveal areas. The Purkinje phenomenon, the alteration ol 

 optical white equations by the state of dark adaptation, the 

 colourless interval for spectral lights of increasing intensity, 

 the different phases of the after-image, all exist, not only in the 

 para-foveal, but also, only gradually diminished, in the foveal 

 region. 



Chemical Analogy. — The visual purple gives a curve which is 

 very similar to that of many other photo-chemical substances. 

 We know that with photo-chemical substances the chemical 

 effect is not proportional to the intensity of the light; a different 

 curve is obtained with weak light from that which is formed 

 with light of greater intensity. It is reasonable, therefore, to 

 suppose that the visual purple which is formed by the pigment 

 cells under the influence of a bright light would be somewhat 

 different in character from that which is formed in darkness. 

 Again, from the chemical analogy which I have just given, even 

 if the visual purple were of the same character we should not 

 expect similar curves with different intensities of light. It is 

 probable that both factors are in operation. This deduction 

 gives an explanation of the Purkinje phenomenon. Not only is 

 the visual purple decomposed and regenerated in daylight, but 

 light is plainly a stimulus for its regeneration. 



The Varying Sensibility of the Fovea (34, 35, 36). — At one 

 moment the fovea appears the least sensitive portion, and at the 

 next moment may be the most sensitive portion of the retina. 

 Helmholtz, whilst recording the fact, confessed that he was 

 quite unable to suggest an explanation. The following simple 

 experiment shows this fact : 



On opening an eye on awaking in the morning and looking 

 at the ceiling, the central portion is seen as an irregular, circular, 

 rhomboidal, or star-shaped black spot. On closing the eye 



