296 Prof. J. Joljona 



vision in the rods fell upon this area. I assume that the* 

 image of a candle flame, which is 1500 metres distant from 

 the eye, is f ocussed upon the central foveal area and effectively 

 covers this. We now have twelve electrons formed per 

 cone per second. It seems safe to conclude that at this 

 distance the fovea would appreciate the candle-flame. 



Now it is very evident that enormously greater numbers 

 of electrons might be generated in the sensitiser bathing the 

 cone without risk of overlapping of the stimuli. For not 

 only must we assume that but a small percentage of the 

 received quanta is restored to the cone in the form of what 

 we may call " characteristic " electrons (i. e. those possessing; 

 speeds near the maximum speed ; in other words, carrying 

 the quantum proper to the wave-length), but we must also 

 bear in mind that the work of each electron occupies at most 

 but a very small fraction of a second. 



We have, indeed, arrived at the weak point of the 

 arrangement — its prodigality. Accordingly, we find that 

 Nature, driven to adopt the external disposition of the 

 sensitiser in order to avoid the confusion prevailing in the 

 rods, proceeds now to improve on her design and to develop 

 the cone so as to obtain the maximum number of high-speed 

 electrons. This is effected by increasing the area of the 

 cone wetted by the sensitiser. For the more the activating 

 light is reduced in intensity per unit area (within limits) the- 

 less the penetration of the ray into the sensitiser, and the 

 greater the number of electrons released at the immediate 

 cone surface. 



Colour vision is in abeyance at very low luminosities. 

 The explanation is that ultimately there are insufficient 

 characteristic electrons to excite the colour sensation. On 

 the other hand, colour vision cannot be excited without the 

 diluting effects of the slower moving electrons appearing. 

 For there must always be many (probably a majority of) 

 slow-moving electrons stimulating the nerve. The sensation 

 arising from these is not characteristic of the frequency 

 which gives rise to them, and a sensation of white light is 

 the result. The colour fails to be "saturated.'*' White 

 sensation is always added to the colour sensation excited by 

 the characteristic electrons. We never experience quite 

 saturated colour sensation. 



It will be gathered that the present theory ascribes 

 quantitative sensitivity to the rods ; qualitative sensitivity 

 to the cones. The difference being mainly referable to 

 the fact that in the one case the sensitiser is located 

 within the nerve, in the other it is located without the 



