Persistence of Vision of Colours of Varying Intensity, 81 



marks one end of the sensitive curve for violet. It will be 

 noticed that the regions" between the transition points X '660 //. 

 and \'4:20fjL and the respective ends of the spectrum are, 

 as before, characterized by purity and simplicity of colour- 

 sensation. The transition point at X'570/jL is again at the 

 intersection of the red and green sensation curves, that is, 

 where the two sensations balance the influence of each other, 

 without sufficient of the third sensation to make itself 

 perceptible. In both figures one can see reasons why the 

 regions between X *420 //, and X *470 jjl, and between X *570 ft 

 and X '660 ft, should be characterized by compound sensations. 

 But it is difficult to understand why the exceedingly complex 

 region between X'470yu, andX*570yu, should exhibit, as it 

 does, persistency curves with only one elevation in the green. 



The occurrence of the four transition points at the boun- 

 daries of the sensation curves cannot be in any sense accidental, 

 and is certainly not intentional, since the transition points 

 were discovered long before any examination of the sensation 

 curves was made in this connexion. 



The cumulative effect of all the experimental evidence 

 presented in this paper seems to warrant most strongly the 

 conclusions already expressed, that there are but three colour 

 sensations, which correspond to red, green, and violet, and 

 that these sensations are primarily excited by hues which lie 

 between the limits set by the four transition points. 



V. The Persistence of Vision of Colours of Varying In- 

 tensity. By Frank Allen, Ph.D., F.E.S.C., Professor 

 of Physics, University of Manitoba, Canada*. 



IN a former communication to the Philosophical Magazinef 

 a method of measuring the luminosity of the spectrum 

 was discussed, which may also be used for the determination 

 of the persistence of vision of colours of varying intensity. 

 The arrangement of apparatus employed in the present in- 

 vestigation is very similar to that described in the paper 

 referred to, and is shown diagrammatically in fig. 1. 



Light from the acetylene flame (A), after concentration by 

 a lens (B), passed through an open sector (90°) of the disk (C) 

 which was rotated by an electric motor, then through two 

 Nicol prisms (D and E) arranged with their principal 



* Communicated by the Author. 



t "A New Method of Measuring the Luminosity of the Spectrum," 

 May 1911. 



Phil. Mag. S. 6. Vol. 38. No. 223. July 1919. G 



