434 Mr, P, Cooper on Accidental Colours, 



have produced a red and a blue shadow, upon the principle 

 already explained. 



But we may procure light of different shades, without 

 having recourse to artificial means for its production ; the 

 fact is, that the light which is reflected towards us in all 

 directions is never uniform in colour, and it sometimes 

 varies so much, particularly in the morning and the even- 

 ing, as to produce very distinct shadows ; these shadows, 

 when they are only two, are always complementary, because 

 the standard for white is formed by the mixed sensations 

 produced by their joint action. 



I have usually slept, during the last six months, in a 

 room with a west aspect overlooking the sea, the window 

 being considerably elevated ; and in the morning, I have 

 uniformly observed, that the reflexion of the light of the 

 rising sun, from the atmosphere, and from the water, pro- 

 duced upon the white blind, which performed the part of 

 a screen, two distinct shadows of the horizontal window 

 bars ; the upper shadow being a pale blue, and the under 

 shadow a faint red. — In this experiment, the standard 

 for white is formed by the light which enters the window 

 from both reflexions, and, as these reflexions form a large 

 angle, the shadows produced by the window bars diverge 

 rapidly, and fall upon the blind quite distinct ; that 

 formed by intercepting the light reflected by the atmosphere 

 being red, and that by intercepting the reflexion from the 

 water blue. 



I have frequently observed the same appearance in the 

 shadows formed by window bars, both vertical and hori- 

 zontal, under other circumstances, particularly, when the 

 light came directly from the atmosphere in one direction, 

 and was reflected from an opposite building in the other. 

 I shall mention only the following instance : — An apartment 

 obscurely lighted by a single window, having a north 

 aspect, receives its light in three different directions ; 

 through an avenue of evergreens in a direct line, by re- 

 flexion from the atmosphere from the right, and by reflexion 

 from the yellow front of a building nearly opposite on the 

 left; this mixed light produces the usual sensation of 

 white, but the window bars, and all opaque objects within 

 the apartment either form distinct shadows of violet. 



