Simultaneous Colour Contrast. 



547 



By moving the candle the relative luminosities of the two shadows can be so 

 •adjusted as to appear similar in brightness. 



The shadow thrown by the candle and which is illuminated by daylight 

 appears blue, whilst that formed by the daylight and illuminated by the 

 reddish-yellow candle light appears yellow. It will be noticed that the 

 shadow of the candle which is illuminated by white daylight is really grey, 

 as it is only illuminated by a white light. Helmholtz and Hering therefore 

 have referred to the blue coloration of the shadow as the subjective blue. 

 I propose to show that the blue coloration of the shadow is relatively 

 objective blue in the circumstances of the experiment. 



It must be noted that the white surface on which the opaque object is 

 standing and which is free from any shadow is illuminated by both daylight 

 and candle light. Though it is still considered as a white surface it is really 

 objectively yellow, to the extent of the added amount of candle light in the 

 total amount of candle light and daylight which is reflected from the white 

 surface, the degree of objective yellowness amounting to the difference 

 between candle light and daylight in the proportion of the two. The blue 

 shadow is therefore relatively blue in comparison to this white surface 

 reflecting both lights when this surface is set up as a standard of white. In 

 the same way the yellow shadow is relatively yellow in comparison to the 

 whole surface. It must be noticed that daylight is not a fixed unalterable 

 white but differs considerably according to the time of day and source ; the 

 light reflected from the sky is much bluer than that of direct sunlight. 



All our estimations of colour are only relative and formed in association 

 with memory and the definite objective light which falls upon the eye. In 

 many of the most striking contrast experiments the colour which causes the 

 false interpretation is not perceived at all : for instance if a sheet of pale 

 green paper be taken for white a piece of grey paper upon it appears rose- 

 coloured, but appears colourless when it is recognised that the paper is pale 

 green and not white. 



If, in repeating the experiment with coloured shadows, the opaque object 

 be placed upon a dull black surface and two pieces of white paper be placed 

 on this surface for the shadows, care being taken that these pieces of paper 

 are the exact size of or smaller than the shadows, these will appear blue and 

 yellow as before. If we now place a small dot on the paper on which there 

 is a blue shadow and having covered one eye keep the other rigidly directed 

 at this black spot whilst an opaque object is placed in front of the candle so 

 that it no longer illuminates the paper or throws a shadow, both pieces of 

 paper will appear white, being illuminated only by white daylight. The eye 

 being still kept rigidly directed on the spot on the paper, the opaque 



