of Simultaneous Contrast- Colour. 167 



one I made on the contrast-colours of the electric flash, when 

 the same colours were distinctly seen apparently at the 

 moment of the discharge. Can one " exercise his judgment 

 and divide between the two images the difference in colour 

 which really exists " in an interval of time which is less than 

 -^ of a second ? 



In the experiment of the coloured shadows cast by the 

 candle and by daylight, these colours are explained by Von 

 Bezold (pp. 152, 153), as follows : — " The spot occupied by the 

 blue shadow is illuminated by the white daylight, the larger 

 white surface by daylight and by candle-light, the other shadow 

 by candle-light only. It might be presumed, therefore, that 

 one of the shadows would appear white, the other yellow. 

 This is not the case, however ; for knowing the surface to be 

 white we still take it to be white after it has really received 

 the yellow light of the candle. Our judgment is led astray 

 regarding white, and hence we believe the place occupied by 

 the second shadow to be blue, although it is actually white." 

 Helmholtz (' Lectures/ London, 1873, p. 267), says :— " Thus 

 in the experiment described above of coloured shadows thrown 

 by daylight and candle-light, the doubly illuminated surface 

 of the paper being the brightest object seen gives a false 

 criterion for white. Compared with it, the really white but 

 less bright shadow thrown by the candle looks blue." These 

 explanations assume knowledge and conditions which are not 

 essential. If this knowledge and these conditions were 

 necessary to see the phenomena then these explanations of 

 the phenomena might be convincing ; but the conditions they 

 assume are not necessary. The following experiments show 

 that there is no necessity at all in " knowing the surface 

 to be white" or to see "the doubly illuminated surface of 

 the paper. 1 " 



The experiment of the coloured shadows cast by the candle 

 and by daylight was arranged behind a screen, so that no 

 one could divine what was there. A tube blackened on the 

 inside went obliquely through the side of the screen and was 

 so adjusted that the circular field of view through the tube 

 was entirely filled by equal portions of the two shadows, 

 which formed two semicircles, one coloured orange the other 

 blue. The two persons on whom I experimented were ignorant 

 of the phenomena of contrast-colour, and, moreover, were 

 misled as to what they would see on looking into the tube, 

 and I was specially careful not to speak to them about colour. 

 These persons were strangers to each other, and neither knew 

 that the other had been the subject of my experimenting., 

 The first observer at once reported : — " I see a circle half 

 yellow and half blue." The other said :— " I see a golden 



