Vision, tracing them to Functional Actions of the Braifi. 255 



affectioia of vision, which I had discovered several years before, 

 and even laid before the pubhc (though in a manner that ap- 

 pears to me very unsatisfactory now), afforded a key to these 

 difficulties. 



It was commonly taught, and, if I am not mistaken, still con- 

 tinues to be so, that in any pre-existing state of the sensibility 

 of the eye, the apparent luminousness of an object is in the 

 direct ratio of the quantity of light from any point of it enter- 

 ing the eye and acting on any point of the retina. Physiolo- 

 gists had been led, by certain appearances, to adopt as a prin- 

 ciple, that the sensibility of the retina is fatigued, impaired, or 

 exhausted by the continued action of strong light; but no one 

 had ever suspected that the sensibility could be augmented in 

 consequence of the action of bright light, or that it could suffer 

 a diminution without the action of light to account for it: yet 

 I ascertained, by a series of unequivocal experiments, that, in 

 certain circumstances, the action of a stro?iger and 'weaker light 

 on adjoining parts of the retina was attended with an increased 

 perception of the colour of the brighter object, and a diminished 

 perception of the same colour in the darker one*. Attention to 



* Some important errors having crept into the account which I formerly 

 published of this affection of sight, I find it necessary, instead of referring 

 to it, to explain here the facts by which the existence of the principle an- 

 nounced in the text appears to be demonstrated. 



Take six pieces of thin white writing-paper, about four inches square 

 each, and having painted them of the principal primary colours, viz. one 

 red, another orange, a third yellow, a fourth blue, a fifth green, and the 

 sixth violet, form them into lubes, to be used as will be directed imme- 

 diately. Form also two other tubes of the same size, — one of them of thin 

 white writing-paper, the other opake and blackened within. 



Now if one of the coloured tubes is applied to one of the eyes, the other 

 eye being naked, and if the tube be strongly illuminated, a white surface, 

 visible to both eyes, will present the following remarkable appearances. 

 The round spot of the white surface that is seen through the tube will ap- 

 pear, not white, but uniformly of the colour that is complementary to that 

 of the tube ; while the rest of it will appear of its proper white colour to 

 the naked eye. Thus if the tube employed is red, the circular t>pot seen 

 through it will appear (jreen; through the yellow* tube it will appear 

 purple ; through the blue tube orange, &c. ; and the power of the colour 

 of the tube to affect the perception of a white surface seen through it may 

 be still further illustrated by using tubes of different shades of the same 

 primary colour. If two such tubes, differing only in degree, be applied to 

 the two eyes at once, and directed to diflerent parts of the white surface, 

 the two circular sjiots thus seen will be as different in the degree of the 

 complementary colour as the tubes are in the primary colour. If we attend 

 to the optical state of the eye during these experiments, it will be found 

 that every part of the retina, cxce[)t a small circular spot towards the 

 middle of it, is acted upon by the coloured light from the tube, and that 

 this circular spot is acted upon by the white light from the surface scon 

 through tiie tube. How is it, then, that we do not see the circular spot 

 white, but of a colour which constantly varies so as to be complemcntiu'y 



