214 Mr. Charles Tomlinson on 



10. When the eye is regarding a wafer upon white 

 ground, the accidental impression can be observed upon 

 another white ground, remote from that upon which the 

 wafer is placed. Thus, suppose a red wafer upon white 

 ground be viewed for a sufficient time, and the eye be 

 directed to the ceiling of the room, or to a contiguous 

 sheet of white paper, a green image of the same form as 

 the red wafer will be perceived. This is due to the per- 

 sistance of the impressions of the retina which retains the 

 image ; and the colour is derived from the white ground 

 on which the wafer is placed, the red of which ground is 

 absorbed by the wafer, and the eye bears a portion of 

 green, equal in extent to the size of the wafer, also by per- 

 sistance of impression for a second or two, upon any other 

 white ground. The accidental image then, is always the 

 size of the red wafer ; and while we regard the latter, and 

 include a portion of the white ground, which it is almost 

 impossible not to do, the accidental image overlaps the 

 primary one, and we see a green segment on the white 

 ground, and the red w^afer is darker in tint because the 

 accidental image is super-posed. If we move the eye away 

 from the wafer upon the white ground, the accidental image 

 moves off the wafer, and sometimes the primary image is 

 lost sight of altogether, and the spectrum alone is retained. 

 This often happens with me, and it maybe that the primary 

 image, in such case, falls upon the base of the optic nerve. 



11. The experiments of M. Plateau may be objected to 

 me, where the accidental colour is obtained when a coloured 

 object, such as the red wafer is placed on a dull black 

 ground instead of white. I admit, that in such cases, a 

 very faint accidental image can be obtained, and do not 

 consider such effects as objections to my theory ; for as the 

 wafer is seen by reflected light the incident light impinging 

 on the wafer, is by the latter deprived of a portion of its 

 red rays, and we see the reflected light yellow and blue 

 forming green. 



12. As it is my wish to strengthen this theory by facts 

 observed under very different circumstances, I may be 

 allowed to introduce the few following experiments, which, 

 apart from all theory, will probably be found not altogether 

 wanting in novelty and interest. 



