RESEARCHES BASED UPON THE THEORY 



225 



the provisional violet sensation would have to be added to it, and the 

 same amount be abstracted from the violet to arrive at the true blue 

 sensation. The green sensation would remain unaltered. 



Having at one end of the spectrum a pure red sensation, and at the 

 other mixed sensations, due to the stimulation of the red and a blue 

 sensation, it remains to isolate the green sensation. Owing to the over- 

 lapping of the curves in the green of the spectrum, due to the fact that 

 this region stimulates all three of the sensations, the effect of the pure 

 green sensation is never experienced by a normal eye. In any colour 

 where the stimulation of all three sensations occurs there must be 

 always an admixture of white light, and we have to search for that point 

 in the spectrum where white alone is added to the green sensation. 



The following diagram, Fig. 61, will show some variations in com- 

 position of a colour that may be met with. The provisional use of a 



a 



G 

 A 



c 



B 



Fig. 61. Diagrams illustrating Abney's method of determining the 

 normal sensation curves. 



violet sensation will not alter the argument, since, as before said, we 

 may replace it by blue and red sensations. The different figures are 

 purely diagrammatic. They are constructed on the supposition that 

 equal heights of line above the base line show the stimulation necessary 

 to give the effect of white light. The scale applicable to each of the 

 three lines is necessarily quite different in the scale of luminosity ; that 

 of the violet in particular is very greatly exaggerated. 



A, B, and C represent colours each containing a sensation of white. 

 Let the stimulation of the sensations be represented by vertical lines. 

 In A we have the red and green sensations of equal heights, but V is less. 

 Drawing a horizontal line through c, aR, bG, and cV, represent equal 

 stimulations, which make white, leaving da and eb equal. We thus 

 have a colour which is made up of a mixture of R and G sensations 

 (RS, and GS), together with white. Now equal stimulations of RS and 



p. c. v. 



15 



