548 



Dr. F. W. Edridge-Green. 



[Dec. 4, 



object is removed so that the candle again throws a shadow on the paper. 

 The shadow thrown by daylight immediately appears yellow and of greater 

 saturation than before, but the shadow thrown by the candle appears white, 

 as before, and without the faintest trace of a blue colour. The conditions 

 are in every way favourable to the development of the blue colour, but none 

 appears because the observer has been able to form a correct estimate of 

 white. If the blue colour were a real subjective coloration caused by the 

 yellow, blue should appear on the shadow from the candle. 



The second experiment which is considered by Hering is that in which a 

 small piece of grey paper is placed in the centre of a large square of 

 coloured paper, and the whole is covered by a thin piece of tissue paper. 

 The centre grey square becomes tinged of the complementary colour of the 

 larger square on which it is placed. This experiment succeeds best when 

 the colour of the ground is green. The grey paper is then tinged with the 

 complementary colour, rose. This experiment, like most others of simul- 

 taneous contrast, has its effect much heightened if successive contrast be 

 allowed to influence the result. In successive contrast the eye becomes 

 fatigued for the colour particular to the rays of light which fall upon it. 



I agree with Burch* that fatigue of the eye for any one colour does not 

 increase its sensitiveness for any other colour. for instance, if the eye 

 be fatigued for yellow the blue of the spectrum is considerably diminished, 

 not increased. This is probably due to luminosity contrast. In the above 

 experiment, therefore, it is very important that the eye should not see the 

 grey square after having observed the green, as in that case it will be 

 tinged with rose colour from successive contrast, because the eye has 

 become fatigued for the green constituent of the white light reflected 

 from it. 



As is well known, the rose coloration is greatly diminished by using 

 black or white instead of grey, or by isolating the grey square by drawing a 

 black line round it. If, however, taking the greatest care that the eye be 

 not moved, by steady fixation of the eye upon a black spot in the centre of 

 the square for 10 seconds, and then looking at a sheet of white paper, a 

 definite after-image is seen, a large rose-coloured square corresponding to 

 the green paper, and a small green square in the centre corresponding to 

 the small grey square, it will be noticed that the rose rapidly encroaches 

 on the green, which disappears, whilst a rapid whirlpool appearance is seen 

 in the centre of the field of vision. This experiment would seem to 

 support strongly the view of Hering that the rose colour is actually 

 subjectively produced on account of the proximity to green. I hope, 

 * ' Phil. Trans.,' B, 1899, p. 5. 



