AND THE THEORY OF LIGHT. 39 



intervals of light aud no-light. I could not adopt New- 

 ton's hypothesis in Halley's experiment in the diving-bell, 

 and say that the glass reflected the green and transmitted 

 every other colour, for that was not the case. It could 

 only be explained on the same principle by which it has 

 been attempted to account for the blue shadow cast by a 

 lighted candle on the window-blind in daylight. In the 

 phenomena just described, refracting substances, water 

 and glass, play a part ; and they afforded only arguments 

 of a secondary kind, since Newton had forestalled the 

 explanation by his theory of refraction, reflection, and 

 transmission. 



60. It will be remarked that green is always associated 

 with a red or a yellow colour in these observations. I accord- 

 ingly attempted to produce green by the mere motion of 

 red light alternating with black, and I think successfully. 

 The green obtained by the motion of the red handkerchief 

 was of this description, but it was not of so decided a nature 

 as I could wish, as I could not make it visible to every one. 

 I do not, however, consider that light should be at first 

 red in order to produce green ; I only mean to assert that 

 a ray of the value of red, when made to vibrate alternately 

 with black or shadow, produces green in a clear atmo- 

 sphere. Another argument bringing us nearer and nearer 

 to the homogeneous nature of light. 



61. I willingly grant, as I formerly remarked, that the 

 combination of two such sensations as red and black, or of 

 yellow and black, producing green, is not apt to strike one 

 as at all remarkable, as we often see the effect produced; 

 but the fact of its being common does not make the 

 investigation of the cause the less necessary. It must be 

 remarked that the mere reduction of the intensity of a ray 

 will not chanoe its colour. 



