290 Mr. Charles Tomlimon on 



scape, the successful imitation of which constitutes good 

 painting. As the shadows of evening lengthen, they will 

 generally be found to be indigo, if received upon white 

 ground. Twilight, assisted by moderate artificial light, is 

 a very favourable time for observing the coloured shadows 

 and opposite tints of nature. By night the shadows cast 

 by artificial lights are seldom capable of being estimated 

 as to colour on account of their intensity, but by twilight 

 they are distinctly seen to be either blue or indigo, and 

 sometimes purple. By the yellow light of a lamp just after 

 sunset, the sky, as seen through the window, is of a very 

 rich indigo : if a sheet of white paper be so arranged as to 

 receive the light of the lamp, and at the same time the 

 natural light from the window, and the finger or other 

 object held so as to throw a shadow upon the paper, two 

 shadows will be obtained, one of bright yellow and the 

 other of a rich indigo : this is a simple and familiar instance 

 of coloured shadows and admits of easy explanation. The 

 paper is illuminated by the yellow rays of the lamp and at 

 the same time by indigo rays from the sky : the former by 

 their intensity and abundance overcome the latter and the 

 paper appears yellow ; a shadow cast by the lamp deprives 

 that portion of the paper occupied by such shadow of the 

 yellow rays; the shadow is therefore illumined by the 

 indigo rays of the sky. A converse arrangement will 

 account for the appearance of the yellow shadow, which is 

 cast not by the lamp but by the natural light which falls 

 upon the ground, illuminated by artificial yellow light and 

 natural indigo light, except where the yellow shadow 

 appears, for there the indigo is intercepted by the opaque 

 object. 



19. The means of obtaining coloured light are various, 

 and may be arranged into four classes ; thus. 



Class \st. The colours of the spectrum. 

 ,, 2nd. Transparent coloured media. 

 ,, Zrd. Coloured flames. 

 ,, Ath. Light reflected from polished coloured surfaces. 



20. Class Ut. On receiving a large and well defined 

 spectrum upon a white screen, or upon the white ceiling 

 of a room, and intercepting a portion of each of the colorific 

 rays in succession, by means of a long narrow strip of white 



