and Coloured Shadows. 433 



had rendered the written part of the paper opaque, while 

 the remainder of it transmitted the red light from the fire; 

 of course, the former reflected the atmospheric light only, 

 and the latter, together with this reflected light, trans- 

 mitted the direct light from the fire ; the eye, turned to- 

 wards the fire, had recognized its light, particularly with 

 regard to the paper, as the standard for white, and, con- 

 sequently, the reflected light of the sun produced the im- 

 pression of blue ; as it would have done, if the eye had 

 been prepared by looking through a piece of red glass. — 

 Another variety of the experiment may be made, by ad- 

 mitting the light of the moon, through a small aperture, 

 into an apartment illuminated by a candle or a lamp; 

 the light of the latter being the standard, the former, of 

 course, will appear blue. 



If the light of the sun and the light of a candle, when 

 reflected by an object usually considered white, give the 

 impression of white light separately, we may readily sup- 

 pose, that, when mixed together, they produce the same 

 impression. In the first of the experiments we have just 

 noticed, the white paper is illuminated with both the light 

 of the sun and the light of the candle, and the reflexions 

 from the mixed lights form the standard for white ; when, 

 therefore, any part of the paper is seen illuminated with 

 one of these lights only, it appears to be of the prevailing 

 colour of this light compared with the mixed light ; thus, 

 the light from the atmosphere appears blue, and the light 

 from the candle red. 



The same explanation applies to all other cases of coloured 

 shadows ; but to produce the full effect, it is necessary, 

 that the intensity of the two lights should be nearly equal, 

 and that they should fall upon the screen in such different 

 directions, that the shadows of any opaque object, formed 

 by the different lights, may be separated. — In the last of 

 the experiments before noticed, the light of the moon was 

 admitted by a small aperture, and, consequently, the light 

 of the apartment, which formed the standard, was not per- 

 ceptibly affected by it ; but, if the two lights had been 

 brought to nearly an equal state of intensity, the mixed 

 light would have formed the standard, and an opaque 

 object, at a proper angle to tbeir different directions, would 



VOL. IV. 2 F 



