AND THE THEORY OF LIGHT. 29 



lated white ray^ if I may use the expression, reflected from 

 the shadow, produces blue in its intervals of pulsation on 

 the black ground. Painters, as well as other observers, 

 have often remarked that black seen through white is blue. 

 This, however, is not the case, in their acceptation of the 

 term, as the blue and grey shadows on snow prove. At 

 present I am not only arguing for the necessity of intro- 

 ducing into the theory of colour a negative term^ namely, 

 shadow or penumbra, but at the same time the equally 

 necessary element, lengthened intervals in the vibratory 

 motion. 



46. It is by such observations that we obtain arguments 

 relating to the colour of the sky, as well as arguments for 

 the necessity of introducing a new term into our researches 

 on the cause of colour. From these observations I con- 

 clude that blue is produced by a sensation or feeling of no- 

 light in the intervals of pulsation between one wave of 

 light and another. 



47. I know that scientific readers will be getting impa- 

 tient at these arguments, and it will be objected that I am 

 even departing from my own premises ; that I am neither 

 proving that colour is formed by reflecting light on the 

 "confines of shadow," as Newton afiirmed could not be 

 done, nor am I proving that shadow has anything to do 

 with the formation of colour. For to what do all these 

 arguments tend, if they tend to anything, but to demon- 

 strate the length of a wave — that the darkness I speak 

 of is the hollow of the wave, or the interval between the 

 crest, apex or summit of one wave and another, which 

 apex is the light? 



48. That colour cannot be produced by reflecting light 

 on the "confines of shadow" in Newton''s acceptation of 

 the term, or in that of the ancients, I admit; but that 

 shadow is necessary to the formation of colour is also true, 

 although in a different acceptation from that of Newton. 



