260 CAPTAIN W. DE W. ABNEY ON THE COLOUR 



this reason to be accepted that the colour is due to only one sensation. It might be 

 due to two or three sensations if they were stimulated in the same proportions along 

 that region, and if the identical colour could be produced by the combination of other 

 colours. Experiment shows that a combination of two colours will make violet under 

 certain conditions, and that instead of a simple sensation of violet we have in this 

 region a blue sensation combined with a large proportion of red sensation. The proof 

 of this and the estimation of the percentage composition of the violet will be given 

 subsequently. It may, however, be here stated, that if we know the percentage 

 composition we may provisionally use this part of the spectrum as if it excited but 

 one sensation, and subsequently convert the results obtained with it into the true 

 sensations. Thus in calculating the percentage of red in any colour, that existing in 

 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. In the first part of this paper the provisional violet 

 sensation will be employed and the necessary corrections subsequently made. 



In using the violet it must be recollected that the colour is usually contaminated 

 with the white light which illuminates the prism or grating, and that such illumina- 

 tion may be very appreciable at a part of the spectrum where the luminosity is very 

 small. White light must therefore be cut off as far as practicable, and by use of an 

 absorbing medium such as blue glass coated with a gelatine film dyed with a blue dye 

 this is attained. The use of a second prism in front of the spectrum is inconvenient 

 though effectual. 



(III.) Possible Mixtures of Sensations. 



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 overlapping 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, though 

 presumably it is by what are termed the red-blind of the Young Theory. 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. 1, will show the variations in composition of a colour 

 that may be met with. The provisional use of a 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 show the stimulation necessary to give the 

 effect of white light. The scale applicable to each of the three lines is necessarily 

 quite different to the scale of luminosity, that of the violet in particular is very 

 greatly exaggerated. A, B, &c., mean that colours may exist each containing a 



