40 On Coloured Shadoxvs. 



Mr Trechsel, in consequence, proposes to distinguish two 

 sorts of coloured shadows, one of which may be termed objective, 

 and the other subjective. Among the former would range them- 

 selves, \st. The shadows, whose bluish colouring is owing to 

 the reflection of the day light ; 9.d, The shadows that are co- 

 loured yellow by the direct light of a candle ; 3t?, Those which 

 are obtained from the reflection of the light by a neighbouring 

 coloured body. To' the subjective shadows would be referred 

 those which are produced in the light coloured either by pris- 

 matic decomposition, or by its transmission through coloured 

 glass. In this latter class would also be placed the remarkable 

 phenomenon of the coloration by day-light in the camera ob- 

 scura, and some other similar phenomena. 



The shadows, whose colouring is produced by direct or ob- 

 jective means, do not require further explanation ; but the case 

 is different with those whose colouring is only subjective. 

 " With regard to these latter," says Mr Trechsel, " Mr Grot- 

 thuss's hypothesis appears to me the most probable. It accords 

 not only with ordinary observation, but also with the experi- 

 ment of the camera obscura which has -been described above, 

 and which was not known to Mr Grotthuss. According to this 

 author, when our eye receives the impression of any colour 

 whatever, for example, orange light, transmitted in large quan- 

 tity, the sensibility of the organ for this light is diminished, and 

 perhaps the sensibility for the complementary blue light in- 

 creases. If we now make the day light, or any other white 

 light, fall upon a shadow projected in this coloured light, or 

 simply upon a ground tinged with this same light, the orange 

 ray disappears subjectively of the day-light, and we then only 

 perceive the united sensation of the other rays contained in the 

 fasciculus, rays which, by their combination, produce a greenish 

 blue tint, complementary of the orange in the scale of Newton. 



No doubt can be entertained of the subjectiveness of the phe- 

 nomenon of the camera obscura, which I have already several 

 times mentioned, if it be brought to mind that the day-light 

 sometimes produces the red tint in it, sometimes the green, ac- 

 cording to the colouring of the ground. Another experiment 

 may be added, which, although not new, is yet not the less 

 striking. Let two candles be placed^ so as that two shadows 



