36- On Coloured Shadows 



and the farther may the opaque body which projects them, the 

 hand, for instance, be removed from the white surface which 

 receives them. The distance, therefore, cannot be assigned at 

 which the opaque body should be from the surface. According 

 lo the greater or less intensity of the light, this distance may 

 vary from five or eight feet to as many lines. At the moment 

 of twilight, or in very dark days, the end of the finger from 

 which the shadow projects, requires to be held at the most two 

 or three lines from the white surface. 4<th, The same opaque 

 body projects shadows variously coloured, according as the sur- 

 rounding surfaces, such as the walls of the chamber, or the 

 clouds, if it be in the open air, reflect one colour or another. 



Coloured shadows also form in a light, coloured by refraction 

 or by reflection. This colouring of the shadows, however, does 

 not take place, if the light so modified penetrates into a chamber 

 otherwise perfectly dark, for in that case the shadows are black. 

 The more intensely the light is coloured, the more distinct is 

 the tint of the colour. 



Lastly, the artificial light of a candle, combined with that of 

 the sun, gives rise to coloured shadows. Thus, according to 

 Rumford's experiments, if, in the day-time, the shutter of a dark 

 room be opened about half an inch, and there be placed upon a 

 table a lighted candle (situated in such a manner, that its rays 

 falling upon a piece of white paper, which is presented to it, as 

 well as to the opening of the shutter, make with those coming 

 from this opening an angle of about forty degrees), and the 

 finger be then held at the distance of two or three inches before 

 the paper, this opaque body will project two shadows, of which 

 that proceeding from the day-light will be yellow, and that from 

 the candle-light of a very beautiful blue. In proportion as the 

 finger is carried nearer the candle, the blue will become deeper, 

 and the yellow fainter, and the contrary will take place if it be 

 removed from the light. 



Such being the facts to be accounted for, Mr Z. proposes to 

 establish, a priori, that the shadows produced by the interception 

 of a coloured light must also be coloured. " It is known," 

 says he, " that, in the solar spectrum, the white light of the sun 

 is decomposed into coloured rays ; on the other hand, the sha- 

 dow produced by the interception of white and undecomposed 



