AND THE THEORY OF LIGHT. 11 



change of colour was the effect of the pulsations of light 

 from the red wafer alternating with the sensation of black. 

 But there was a difficulty even here, for the impression of 

 red was not changed when the other eye was completely 

 shut. In answer to this I argued, that there could be no 

 sensation where there was no agitation of the retina. The 

 black wafer could not have been seen where it was, unless 

 the rest of the retina had been agitated with light. In 

 an eye which is shut, every thing is in a state of repose, 

 and there is nothing to interfere with the sensation pro- 

 duced on the other eye, for there is no contending sensa- 

 tion. It is the mixing or uniting of these independent 

 sensations which produces the sensation of change of 

 colour. Black then may be considered as a colour in so 

 far as it is a sensation. 



1 7. If the two retinas are intellectually one, and if each 

 individual retina produces its own impression, it is evident 

 that the number of vibrations on the sensorium must be 

 diminished one half if only one eye is used. The two 

 spots are perceptionally or sensationally one spot, but 

 when considered intellectually or philosophically, the per- 

 ception is discovered to be a compound of two coordinate 

 sensations, and being coordinate they are sensationally 

 one. 



18. But it may be argued that the wave of red light 

 suffered a change before entering the sensorium, for the 

 fibres of the one eye being in motion and the other at rest 

 or quiescent, a diminution of velocity at the junction would 

 be the consequence. This should not affect the length of 

 the impinging wave, which is considered as the cause of 

 colour, if red is an unchangeable element ; but even if it 

 did, this supposition would not account for there being no 

 change when the one eye was completely shut, although 

 the same process must still go on. 



19. The unavoidable inference from this experiment 



