A Quantum Theory of Colour Vision. 



219 



Plate 16. 



Yig. 9. — Mount Elgon Initiation Rites. Before the initiation ceremony the young men 

 perform dances at various villages and receive gifts for the circumcision feast. On 

 the day of the ceremony, before the event takes place, they go round the village 

 and drive out hostile ghosts supposed to be lurking round. 



rig. 10. — Mount Elgon. During the operation of circumcision the patient stands with 

 arms extended upwards, and must show no sign of timidity. Should he either 

 utter a sound of pain or move a muscle he is branded a coward. Shame will force 

 such a youth to commit suicide rather than face his companions afterwards. 



A Quantum Theory of Colour Vision. 

 By J. JoLY, Sc.D., F.R.S. 



(Eeceived April 8, 1921.) 



In a paper on the subject of the Quantum Theory of Vision, issued in the 

 'Philosophical Magazine' (February, 1921), I dwelt on the view that the 

 sensation of light is in every case stimulated by the action of photo-electrons 

 set free in the retina. Further, the energy of the photo-electron being pro- 

 portional to the frequency of the light, the strength of the stimulus produced 

 is the all-sufficient origin of colour sensations. That colour is entirely a 

 cerebral phenomenon is evident. Light, visible and invisible, consists of a 

 uniformly graduated series of wave motions or energies. There is nothing to 

 distinguish one part of the spectrum from another save the difference of 

 wave-length or frequency. But objects in Nature react differently towards 

 these waves : absorbing some, reflecting others, and so the selective effect of 

 natural objects towards light has discovered to the organism a means of 

 improving on mono-chromatic vision : a means of distinguishing objects by 

 their selective absorption and reflection. Our colour sensations were deve- 

 loped solely for this purpose and solely under the influence of the light 

 reflected by natural objects. Hence, a limited number of fundamental sensa- 

 tions being the simplest, if not indeed the only, way of securing the desired 

 end, we would expect that these sensations would be developed so as most 

 effectively to interpret the frequencies met with among natural objects 

 reflecting solar light. The evolutionary development of three highly deve- 

 loped colour sensations according to the extreme and mean regions of the 

 spectrum is the result. Colour sensations, i.e. (white), red, green, and blue 

 were evolved, whereby the whole gamut of the spectrum can be dealt with. 



VOL. xcii. — B. s 



