116 P. C. on the Colours that enter into the [Aug. 



eye is sufficiently impressed, and upon then turning to a 

 white object, such as a sheet of white paper, I see the image 

 of the aperture in the complementary colour. 



My principal inducement to make the experiment in this 

 way was, that it gave me a command of colours which I 

 had an opportunity of analyzing by other methods, and 

 some of which I could not readily procure by any other 

 means. I soon, however, found that it had other advan- 

 tages, among which was the ready production of more 

 vivid spectra than those procured by the usual method. 



In the course of these experiments, when the white side 

 of the card was turned towards the eye, I observed the 

 complementary colours playing about the margin of the 

 aperture; and having before suspected that this appear- 

 ance, upon which an eminent philosopher has founded a 

 new theory of accidental colours, was caused by an invo- 

 luntary motion of the eye, which must necessarily bring 

 some of the impressed part of it in the direction of the 

 white paper, and thus produce the complementary colour, 

 I varied the experiment, either by moving the card or my 

 eye, and found that the breadth of the fringe corresponded 

 with the motion, so that at length it became enlarged to 

 the full size of the aperture, which was thus faithfully 

 represented on the card. I perceived that the colours of 

 the spectra produced in this way were much more vivid 

 than when they were transferred to a distant object, and 

 I have since usually adopted this mode of making the ex- 

 periment. 



In order to satisfy myself that this is the true explanation 

 of the appearance, I turned the black side of the card 

 towards my eye, and when all the light except that which 

 passed through the aperture was excluded, by looking 

 through a tube, the inside of which was blackened, the 

 complementary colour did not appear on the edges. 



If the coloured object be placed on a coloured ground, 

 the colour seen upon the margin corresponds with the 

 accidental colour produced by other parts of the same 

 ground. If, for instance, a red seal is placed upon yellow 

 paper, the accidental colour seen upon the same paper is a 

 yellow green, corresponding precisely with the colour upon 

 the edges, which if the seal is withdrawn appears to form 



