252 Mr. T. Smith's Investigation of certain Phanomena of 



Exp. 3. When both eyes, directed to P, are shaded by 

 means of the screens A B and C D, the two images S' and S" 

 appear vohite. The same is true when both eyes are exposed 

 by withdrawing the screens ; but a difference in the degree of 

 their whiteness occurs in these two circumstances, which is 

 too important to pass unheeded. When both eyes are shaded, 

 havino; attended well to the degree of whiteness which the two 

 images of S assume, suddenly withdraw both of the screens ; 

 and though the two images will still seem white, yet they will 

 appear sensibly darker than before, as if a thin cloud had been 

 suddenly interposed between them and the source of their illu- 

 mination. By interposing the screens again, their lustre is sud- 

 denly and permanently restored. 



Exp. 4. To ascertain through what portion of the retina 

 this extraordinary affection of sight occurs, I made the image 

 of the slip of white paper S traverse the retina of the exposed 

 eye, from the borders of the bright image ^y to o and the cor- 

 responding parts of the shaded eye; but 1 could not observe 

 that the green appearance to the exposed eye and red appear- 

 ance to the shaded eye, was less at any one point than another. 

 If the bright image fell on the centre of one retina, every part 

 of the retina around that part, both in it and the other eye, 

 appeared, as far as the image of S remained perceptible, under 

 the operation of this affection. 



These four experiments illustrate the principal facts that 

 require to be kept in view. It is obvious that the green ap- 

 pearance of the white paper S to the exposed eye, and the 

 complementary red colour which it assumes to the other eye, 

 are phaenomena which admit of explanation in two ways ; for 

 they may be accounted for either by an existing excess of sen- 

 sibility to the apparent colour, or by a defect of sensibility to 

 its complementary colour. The first step, therefore, which I 

 took in this stage of the inquiry into the causes of the phaeno- 

 mena, was, to investigate the precise nature of the effects pro- 

 duced by the action of the bright light F 07i one eye. For this 

 purpose I made the following experiments. 



Exp. 5. Having painted a narrow piece of white paper as 

 neai'ly as I could of the green colour which the white slip S 

 assumed to the exposed eye in the first experiment, I substi- 

 tuted it in the place of S ; and when my two eyes were directed 

 to P, I observed that the left-hand image S', which was seen 

 by the exposed eye, appeared of a deep green colour, and the 

 right-hand image S", seen by the shaded eye, of a bright 

 •whitish colour. When the two images were made to coalesce 

 partially by directing the eyes to P', the part be, in which 

 both colours united, appeared of that degree of green colour 

 which I had given to S. 



