AND THE THEORY' OF LIGHT. 9 



attempted an explanation^ to the best of my ability, ac- 

 cording to the received theories of light, but could not 

 satisfy myself. 



1 1 . The phase of the experiment to which I wish parti- 

 cularly to direct attention is this : the image of a red wafer 

 being on one eye, the image of a black wafer on the other 

 eye, the resultant of the two, in a favourable light when 

 the one is made to overlie the other, is neither red nor 

 black, but some other colour. Why another colour ? Why 

 not red ? According to the corpuscular theory black is not 

 an active principle, it is a negation of colour, and should 

 produce no effect on the red. It exacts too much from 

 our faith to believe that two molecules combine some- 

 where beyond the retina to effect a change of colour before 

 reaching the sensorium. And there is no change in the 

 process of refraction of the red to account for the change 

 of colour or to help us to an explanation. 



12. On endeavouring to find an explanation, according 

 to the undulatory theory, I found the same difficulty. 

 Black is not an active principle on the undulatory theory 

 any more than on the corpuscular. If the quality of a 

 colour depends on the length or form of a wave, there is 

 nothing apparently in this experiment to alter the form of 

 the wave, nothing to lessen the number of vibrations, or 

 to reduce its intensity. Why then a change of colour, let 

 the colour be what it may ? 



13. Trying an explanation of the phenomenon by the 

 theory of complementary colours, there were difficulties 

 equally great to surmount. The retina is not fatigued 

 although the muscles of the eye are,* and there is no re- 

 actionary force to account for the colour on the commonly 

 received interpretation of these optical phenomena ; so that 

 I failed to find a solution in whatever way I attempted it. 



* The eyes may be injured very much in making this experiment. 

 SER. III. VOL. I. C 



