xxvii.] VISION/ 289 



of the rest of the paper illuminated by the 



candle. 



This effect of contrast is subjective, not 



objective. To shew this, blow out or place a 



screen before the candle so as to do away 



with the candle-light, The place formerly 



occupied by the shadow now appears white. 



Through a small black tube, e.g. a piece of 



black paper rolled up, with so small a bore 



that the whole field of view lies within the 



shadow, look at the centre of the area formerly 



occupied by the shadow. It still of course 



appears white. While looking, let some one 



light the candle, or remove the screen. No 



change will be visible to the observer looking 



through the tube. 



If the tube however be directed partly on 



the area of the shadow, and partly outside, 



the blue tint of the shadow will, on the candle 



being lighted or the screen removed, become 



apparent as before. 



The daylight-shadow heightens the effect on 



the candle shadow, but may be dispensed 



with. 



In place of sunlight and candle, two coloured 



lights may be used. 



In the above experiments ( a, b, c) avoid look- 

 ing at the colours too fixedly and for too long a 

 time. Otherwise the results will be modified by 

 after-images. 



20. Visual Judgements. Measure on a card two 

 L. 19 



