294 Mr. Charles Tomlinson on 



white paper, a shadow formed by intercepting a portion of 

 the coloured reflected light, will be the accidental colour 

 of that light. Surfaces of burnished gold, polished copper, 

 silver, steel, brass, &c., are instances, as also bright coloured 

 varnished papers. These methods are, however, imperfect. 



24. From what has been already stated of my theory, its 

 application to coloured shadows will be readily perceived. 

 If white ground be illuminated by coloured light, natural 

 light being also present, a shadow introduced occupies a 

 portion of the white ground, which would otherwise be 

 coloured, and we see the shadow white, with the exception 

 of the colour which illuminates the ground. Thus, if the 

 source of coloured light be a green flame, and a sheet of 

 paper be placed so that it be illuminated by the green flame, 

 and at the same time by light from a second source inde- 

 pendent of the first, an opaque object placed between the 

 flame and the paper intercepts a portion of the former, and 

 restores that part of the paper occupied by the shadow to 

 its original whiteness ; but this shadow which would be 

 white is surrounded by green, which, according to the 

 principle of homo-chromatic attraction, absorbs green from 

 the white shadow, and red, therefore, alone remains. 



25. Coloured shadows cannot be produced, unless other 

 light be present than the coloured light which illuminates 

 the paper. Thus, when Brewster's mono-chromatic lamp 

 is employed in an apartment where there are no other 

 sources of light, no accidental shadows are obtained. The 

 pale blue or indigo flame aff'orded by pure carbonic oxide 

 may be made to illuminate a sheet of white paper, but if 

 the room be deprived of all other light, it is in vain that 

 we attempt to produce the accidental shadow. These facts 

 eminently support, I think, the theory I have propounded. 

 A sheet of white paper is no longer white if illuminated 

 only by coloured light : the paper must evidently reflect 

 rays to the eye identical with those whence it receives light, 

 which is, we suppose, in this case, a coloured flame. A 

 shadow formed upon the paper, by intercepting a portion 

 of the light of such flame, is evidently a deprivation of light 

 and nothing more. Hence, such a shadow, if not black, 

 is, at least, very dark, and can have no colour. If now we 



