AND THE THEORY OF LIGHT. 15 



How is this colour produced in ordinary cases in 



nature ? 



Experiment II., or Card Experiment. 



24. If we take a piece of white Bristol board and paint 

 on it with lamp black any figure whatever^ and hold it up 

 between us and the lights the figure will appear purple in 

 place of black. If the board is then inclined^ so as to 

 allow the side nearest to us to reflect a little more of the 

 light entering the room, the purple will now be changed 

 into a faint blue. This demonstrates, not as Newton 

 said that there is blue in black, when he found that he 

 could obtain blue of a certain order by reflecting black on 

 white, but that black or a shadow and the reflected light 

 plus the light transmitted through the card produced the 

 impression of a faint blue. For we have here black in one 

 light appearing as purple, and the same black in an addi- 

 tional light as pale blue. 



This experiment is adduced as another argument that 

 shadow has much to do with colour, and as a step in the 

 process of reasoning by which I arrived at what I consider 

 the mechanical demonstration of the cause of colour. 



25. I am aware that this and similar cases are consi- 

 dered as anomalous, for which special explanations must 

 be framed; and solely because it has been considered 

 demonstrated that white light is heterogeneous in its 

 nature. Por instance, it will be said of the first phase of 

 this experiment that there was a purple in the lamp black, 

 and that the purple only was transmitted, and in the other 

 case that the blue only was reflected, the other rays being 

 absorbed. This is merely, however, ringing a number of 

 changes on words without any positive or experimental 

 facts from which to argue. Such arguments have one 

 advantage ; they are easy, and can surmount any diffi- 

 culty. 



