38 On Coloured Shadows. 



to them others which he deemed necessary for completing the 

 examination of the question. 



He contests first the fundamental proposition of the preceding 

 theory, remarking, that if the shadow is the local absence of light, 

 the light may be partial or total, without this necessarily pro- 

 ducing any change in the nature of the shadow in question. 

 The interception of the light can only produce shade, but not 

 coloured shade, at least unless the colouring be imparted to it 

 from some other source. 



The experiment which we have related, in which black sha- 

 dows are seen to form in a light decomposed by the prism, or 

 coloured in any other manner, provided the chamber be dark, fur- 

 nishes another argument against this hypothesis. Mr Trechsel has 

 obtained nearly the same results as M. Zschokke, with reference 

 to this class of shadows. He has made his experiments by pass- 

 ing the light of the sun or of a candle, through disks of coloured 

 glass*, as well in a lightened apartment as in a large drawing 

 camera obscura, in which the object-glass was replaced by 

 differently coloured glasses, in such a manner as to produce the 

 tint desired upon a piece of white paper placed in the bottom. 

 In the camera obscura the shadows are always black, if the 

 light be excluded from all parts ; they immediately become co- 

 loured when some other light is allowed to penetrate, and the 

 tints which they then assume are always complementary of those 

 of the light transmitted ; thus in the red hght the shadow is 

 blue or greenish, in the green light it is pale red, &c. 



These observations naturally lead us to conclude, that the 

 colouring of shadows does not depend upon the nature of the 

 intercepted light, but rather upon the day -light which mingles 

 with them. This conclusion is enforced by an experiment which 

 does not appear to have been hitherto made, and which is of 

 great weight in the question. If the day light, introduced with 

 proper management, for example, by raising the curtain a little, 

 be made to fall upon the bottom of the camera obscura, whfen it 

 is coloured green by an object-glass of that colour, the place 



• The author had for this purpose large squares of coloured glass furnished 

 by the brothers Miiller, young artists known by their success in the attempts 

 which they have made, especially of late, to rediscover the art of painting 

 glass, which had been lost for several centuries. 



