12 MR. J. SMITH ON THE ORIGIN OF COLOUR 



then is that colour is produced by intervals of light and no- 

 light. Had there been no intervals in the motion of the 

 incident red ray, it is scarcely possible to conceive that 

 there would have been a change of colour. But a change 

 of colour being perceived, it is but natural to suppose 

 that this change must be caused by the colour or no-colour 

 of the spot against which an object is seen in the intervals 

 of pulsation of the light by which it is made visible. 

 Doubtless, as the number of pulsations of the red was not 

 diminished, and only half of the sensatory nerves were ex- 

 cited, there may be two suppositions ; we may either 

 infer from this experiment that the number of vibrations 

 to produce green is only half of those of red, or that only 

 one half of the force is necessary to produce the one colour 

 which it takes to produce the other. Indeed both sup- 

 positions may be true. But I am rather inclined to think 

 that this experiment proves that each particular colour does 

 not depend so much on the number of vibrations in a given 

 time, the usual solution, as on the necessity of two inde- 

 pendent sensations. For although the colour is changed, 

 the vibrations from the red object are not changed. When 

 I look at a red object the vibrations continue the same, 

 and the impression is not altered ; but if I take a red object 

 and make it produce vibrations alternately with a black 

 one, I get the sensation of green, which is exactly what 

 takes place in this experiment, and which I wish to im- 

 press, although there is a difSculty in mastering the physi- 

 cal process. There is this difference in the two processes ; 

 in the wafer experiment the vibrations of red are not re- 

 duced, but when I make red vibrate alternately with black, 

 I reduce the number of vibrations of the red ; but the 

 same process goes on, for the red and black, alternating 

 with each other, become virtually two retinae looking at 

 different objects with exaxlar vision. 



20. With this experiment then before me, and these 



