ON THE RELATION OF OPTICS TO PAINTING. 119 



ground of blue, and a rose-red tint where they im- 

 pinge on green, provided that none of the light 

 collected from the blue or green can fall upon the 

 grey. Where the sun's rays passing through the green 

 leafy shade of trees strike against the ground, they 

 appear to the eye, tired with looking at the predomi- 

 nant green, of a rose-red tint; the whole daylight, 

 entering through a slit, appears blue, compared with 

 reddish yellow candle-light. In this way they are re- 

 presented by the painter, since the colours of his pic- 

 tures are not bright enough to produce the contrast 

 without such help. 



To the series of subjective phenomena, which 

 artists are compelled to represent objectively in their 

 pictures, must be associated certain phenomena of 

 irradiation. By this is understood cases in which 

 any brignt object in the field spreads its light or 

 colour over the neighbourhood. The phenomena are 

 the more marked the brighter is the radiating object, 

 and the halo is brightest in the immediate neighbour- 

 hood of the bright object, but diminishes at a greater 

 distance. These phenomena of irradiation are most 

 striking around a very bright light on a dark ground. 

 If the view of the flame itself is closed by a narrow 

 dark object such as the finger, a bright misty halo dis- 

 appears, which covers the whole neighbourhood, and, at 

 the same time, any objects there may be in the dark 



