234 RECENT PROGRESS OF THE THEORY OF VISION. 



These examples show very plainly how independent our 

 judgment of colours is of their actual amount of illumination. 

 In the same way, it is scarcely affected by the colour of the 

 illumination. We know, of course, in a general way that 

 candle-light is yellowish compared with daylight, but we only 

 learn to appreciate how much the two kinds of illumination 

 differ in colour when we bring them together of the same in- 

 tensity as, for example, in the experiment of coloured shadows. 

 If we admit light from a cloudy sky through a narrow opening 

 into a dark room, so that it falls sideways on a horizontal sheet 

 of white paper, while candle-light falls on it from the other side, 

 and if we then hold a pencil vertically upon the paper, it will 

 of course throw two shadows : the one made by the daylight 

 will be orange, and looks so ; the other made by the candle-light 

 is really white, but appears blue by contrast. The blue and the 

 orange of the two shadows are both colours which we call white, 

 when we see them by daylight and candle-light respectively. 

 Seen together, they appear as two very different and tolerably 

 saturated colours, yet we do not hesitate a moment in recognising 

 white paper by candle-light as white, and very different from 

 orange. * 



The most remarkable of this series of facts is that we can 

 separate the colour of any transparent medium from that of 

 objects seen through it. This is proved by a number of experi- 

 ments contrived to illustrate the effects of contrast. If we look 

 through a green veil at a field of snow, although the light re- 

 flected from it must really have a greenish tint when it reaches 

 our eyes, yet it appears, on the contrary, of a reddish tint, from 

 the effect of the indirect after-image of green. So completely 

 are we able to separate the light which belongs to the trans- 

 parent medium from that of the objects seen through it. 2 



The changes of colour in the last two experiments are known 

 as phenomena of contrast. They consist in mistakes as to local 



1 This experiment with diffused white day-light may also be made with 

 moonlight. 



2 A number of similar experiments will be found described in the author's 

 Handluch der physiologischen Optlk, pp. 398-411. 



