228 Measurement of Colour produced by Contrast. [June 21, 



of the square and making the black interval between the colour and 

 A), when the colour was saturated white appeared perfectly white, 

 whilst if dilute just a shade of contrast colour was visible. 



By placing a diluted coloured space in contact with a pure white 

 space which was in its turn in contact with a saturated colour, 

 it became possible with several colours to make the diluted colour 

 appear white in contrast to the contrast colour itself. With red 

 this became impracticable, and for a reason which will be apparent 

 in a paper which I propose shortly to communicate. 



In this paper it is not intended to include the changes made in 

 undiluted colours by contrast with white or other undiluted colours, 

 or between colour mixtures. It may, however, be said that except 

 for the red and the violet there is a tendency for the two colours to 

 become more widely separated in the spectrum. Thus with red and 

 yellow the red remains of the same hue, but appears slightly more 

 saturated, whilst the yellow appears greener. This would necessarily 

 follow from, the contrast colours produced on a white stripe by satu- 

 rated colours. 



The reasoning given to explain contrast colours on the Toung 

 theory, or on that of Hering seems insufficient, and very hypothetical. 

 I would, however, call attention to a curious phenomenon which 

 General Festing and myself described incidentally in " Colour Photo- 

 metry, Part III." When getting the final extraction of colour 

 from a red ray by a direct comparison with very faint white it was 

 found that when apparently both appeared of the same grey hue, 

 if the white light were increased in intensity, the colour of the red 

 immediately appeared, and it was only after making a comparison 

 for colour with the stronger white that the red colour truly ap- 

 peared to be colourless. In this then'evidently the part of the retina 

 on which the red colour was received was stimulated by the white 

 adjacent to it and to such an extent that the supposed extinguished 

 colour reappeared. Presumably then whilst the retina is excited bv 

 diluted colour, the part on which the pure white may fall may also 

 be excited by it, and not necessarily by the exact complementary 

 colour, since the eye is more sensitive to some colours than to others. 

 Whatever view may be taken of this hypothesis, must not, however, 

 be allowed to detract from the results of the experiments, which are 

 facts recorded and observed with all possible care and after due 

 precautions were taken to avoid error. 



