348 Mr. J. H. J. Poole on the 



vision. When, however, Sir Oliver Lodge independently 

 proposed the same theory, it was thought worth while 

 repeating the experiments more fully, and seeing if any 

 definite evidence in favour of the theory could be obtained. 

 It seems highly probable, it might almost be said certain, 

 that the effect of light on the eye must ultimately be due to a 

 photo-electric action of some kind ; but the real \ oint of issue 

 is whether it can be shown that the photo-electron excited by 

 the light ever gets free of its parent molecule as it does when 

 a metal is illuminated with ulira-violet light, or whether the 

 action is less violent and may only result in disturbing the 

 equilibrium of the electron inside the molecule, and hence 

 altering the chemical properties of some light-sensitive 

 compound in the eye. The latter view would favour such a 

 theory of vision as the Young-IIelmholtz theory, which 

 postulates the existence of three definite and independent 

 colour-sensations in the eye, which might be caused by three 

 different light-sensitive compounds ; but if it could be shown 

 that the former view were correct, a very much more simple 

 and direct explanation of colour-vision can be given, at least 

 from the physical point of view. This explanation rests 

 on the fact that it has been shown that the velocity of 

 emission of a photo-electron is a simple function of the wave- 

 length of the incident light — in fact, that the energy of the 

 electron is given by the equation E = hv—p, where h is 

 Planck's constant, v is the frequency of the light, and p is 

 the work done in getting the electron free from the molecule 

 and will thus be a constant for any particular material. 

 Thus for every different colour in the spectrum an electron 

 of a definite velocity would be emitted, and it is only 

 necessary to suppose that the manner in which the cones are 

 excited depends on the velocity of these electrons, to obtain 

 a possible view of the mechanism of colour-vision. It might 

 perhaps be urged against such a theory as this, that on this 

 view the eye ought always to be able to distinguish between 

 a pure single frequency light and an impure mixture of two 

 different frequency lights, which, as is well known, the eye 

 is in certain cases incapable of doing : e. g. 9 it cannot dis- 

 tinguish between spectrum yellow and a suitable mixture of 

 green and red. However, we could meet this objection by 

 supposing that the resolving power of the cones for photo- 

 electrons of various velocities was not perfect, and that they 

 were only capable of separating the electrons into three main 

 groups, which would correspond to the three primary sen- 

 sations of red, green, and blue: Thus in the case of spectrum 

 yellow, which lies between red and green, the resultant 



