216 Mr. Charles Tomiinson on 



the black disk on the centre of the white board : place the 

 boards back to back as before, so that one eye may see the 

 black and the other the white board, and on making the 

 observation as before (except that instead of viewing the 

 ceiling the. disks must be viewed) the disks will appear to 

 have changed places. Both these illusions, (1) and (2), are 

 very perfect; the effects are immediately obtained, so much 

 so, that many persons to whom I have shown them attribute 

 them to a species of natural magic, rather tljan simple 

 physical experiments. 



(3.) The black board may be employed alone, and the 

 observations made, as has been just described, when the 

 white disk will appear on the other side. 



15. A white disk upon black ground becomes black and 

 a black disk upon white ground becomes white, or in other 

 words, white and black are complementary. These facts are 

 easily explained on the principle of iso-chromatic attraction. 

 A white disk on black ground reflects rays of all the three 

 colours, and is surrounded by black which reflects nothing. 

 The white is absorbed by the black, and we do not see the 

 white disk at all, or we see it with black super-posed, (10) 

 in which latter case, the white is changed into a deep colour, 

 not quite black but approaching to it. A converse expla- 

 nation, of course, applies to the black disk on white ground. 

 The theory of the coloured boards is this : the coloured 

 rays absorb homo-chromatic rays while the rest enter the 

 eye, so that the right eye, for instance, looking along a red 

 board at the ceiling sees green thereon, because the white 

 light from the latter before entering the eye is decomposed ; 

 the red of the board absorbing the red rays from the ceiling 

 and yellow, and blue rays enter the eye. The left eye look- 

 ing along a green board at the ceiling sees red : in this 

 case, the green absorbs yellow and blue rays ; and the red 

 rays are unobstructed in their passage to the eye. 



16. Accidental impressions of colour present themselves 

 to our notice in the majority of cases, under the form of 

 coloured shadows. These phenomena are so extensive and 

 so beautiful that they ought to claim by far the largest 

 share of the attention of any one who would study accidental 

 and complementary colours with ease, and under circum- 

 stances so well adapted for observation ; I shall, therefore, 



