468 



Mr. C. Lloyd Morgan. 



equal increments of sensation increments of colour stimulus in 

 geometrical progression were required, further work was based on the 

 assumption that this empirical generalisation is trustworthy. For 

 convenience of plotting an arbitrary percentage scale was used in each 

 case, so that the curves merely represent the percentages of red, blue, 

 or other stimulus which give equal increments of colour sensation 

 between black and the unmodified colour reckoned as 100. The curves 

 being constructed on similar principles, they are sufficiently indicated 

 by reference to their mid-points, that is, to the stimulus which affords 

 50 per cent, of colour sensation. The following table gives the results 

 for five colours : — ■ 



Mid-point. 



Light yellow on black 13*5 per cent, of yellow stimulus. 



Orange on black 18*0 „ of orange ,, 



Light blue on black 19*0 ,, of light blue stimulus. 



Eed on black 23*0 „ of red stimulus. 



Full blue on black 28'0 „ of blue stimulus. 



Two cases were also taken so as to afford the even sensational 

 shading of white into colour. The results obtained were as follows : — 



- 



Mid-point. 



"White on full blue 25 per cent, of white. 



„ red 30 ,, of red. 



And three cases were taken so as to obtain even shading from one 

 colour into another — for example, red into blue through intervening 

 tints of purple — with the following results : — 



Mid -point. 



Orange on full blue 36 per cent, of orange. 



Yellow on light blue 40 „ of yellow. 



Red on full blue 44 ,, of red. 



The fact that in all these ten sets of experimental results, a curve 

 is obtained based on the principle that equal increments of sensation 

 require increments of stimulus in geometrical progression, materially 

 broadens the empirical generalisation based on the observation work 

 for the shading of white into black. 



Can we not, however, bring the results yet further into line and 

 express them all as portions of a single curve exhibiting the relation of 

 visual stimulus to visual sensation ? 



It is well known — largely through the valuable work of Sir Wnu 

 Abney — that the luminosity of airy colour may be measured by 

 matching it with a grey.* I have thus determined the luminosity of 

 my coloured papers in terms of greys produced by sectors of the black 



* See Abney, 'Roy. Soc. Proc.,' vol. 67, No. 436, p. 118. 



