68 Conjectures respecting the Principles 



have certainly produced appearances of colours or 

 tints, when their pictures are viewed in a proper light 

 and at a proper distance, which we search for in vain 

 upon the canvas. This may well be called the " magic 

 of colouring ;'' for it is in fact calling up, as by en- 

 chantment, and presenting to the mind colours the 

 most pure and vivid, which have no real existence. 



As it might very naturally be suspected that the 

 colours called up by means of shadows owe their exist- 

 ence to something peculiar to shadows, and that similar 

 effects could not be produced without shadows, by 

 means of coloured pigments, to remove all doubts on 

 that subject, I made the following decisive experiment. 



Having found that when a beam of deep red light and 

 a beam of white or colourless light, of equal intensity, 

 arrive in different directions at a plane white surface, 

 and illuminate it, that a blue shadow, nearly approach- 

 ing to green, is called up by the red shadow, I attempted 

 to imitate this experiment with a coloured pigment. 



On the middle of the floor of a spacious room I 

 laid down a very large sheet of black paper, and on 

 the middle of this I placed a circular piece of crayon 

 paper, which, in order that it might supply the place 

 of the illuminated plane surface on which the shadows 

 were projected in my experiments, I covered or col- 

 oured it with such a mixture of red lead {minium) and 

 pure white lead, both finely powdered and well mixed 

 together as brought it to be of the same tint, as nearly 

 as possible, with the surface illuminated by the red and 

 by the white light. I then took two oblong slips of 

 crayon paper, half an inch wide and two inches long 

 each : then, colouring one of them as highly as possi- 

 ble with red lead, in a dry powder, and covering the 



