1894.] Measurement of Colour produced ly Contrast. 



227 



There are such small differences in the wave-lengths of the con- 

 trasts produced by the diluted and saturated colonrs that it may be 

 presumed they are due to error of observation, although each table 

 is derived from the mean of several observations extending over a 

 period of three years. It may be interesting to state that in every 

 case the extremes in the one series embraced the mean value tabulated 

 in the other series, and that in no case did the mean differ from any 

 single observation more than X 2*5. 



There is, however, a very simple means of noting the accord- 

 ance between the contrasts caused by the diluted and saturated 

 colours. With one instrument the contrast caused by the saturated 

 colour was shown on one surface, and with the other that by the 

 same colour, but diluted, on another surface, so that the two could be 

 directly compared. To the eye the only difference between the two 

 was in the amount of dilution of the colour produced by contrast; 

 otherwise they appeared absolutely identical. 



An endeavour was made to ascertain at the same time what dark 

 interval between the white and the colours would prevent the con- 

 trast being appreciable. To do this a cube with a whitened surface 

 was placed as shown on the top of another white surface with a black 

 interval between the two (fig. 3). 



The colour patch was thrown so as to fall only on the cube f, 

 whilst the white beam illuminated the white surface a as well. When 

 the white beam was also thrown on another cube a foot away it was 

 practicable to form an idea of the colour of " a." The effect was 

 curious and interesting. When the black band b was just ^ inch in 

 depth, whilst the white stripe A appeared strongly coloured, a 

 appeared very nearly white, and if by an artifice saturated colour 

 surrounded A it was pure white. If black intervals were placed on 

 each side of A. the colour in A did not disappear, but appeared to be 

 more diluted, probably owing to contrast in the white caused by 

 black, but the colour still remained. If, however, a black interval 

 was on one side of A (that is by placing the shadow against the edge 



Q 2 



