442 M. Plateau's Defence of his Theory of the Visual 



falling on another part of the retina, the same may serve to 

 establish a comparison, and allow you to appreciate the ap- 

 parent alteration that the colour of the first object has under- 

 gone. Now this alteration is by this means rendered ex- 

 tremely sensible : the colour of the paper which has produced 

 a prolonged impression, appears much darker than that of 

 the other. It is therefore to be seen that we must attribute 

 the effect observed by Sir David Brewster to the influence of 

 the ground upon which the object is placed. I will quote, on 

 that subject, a fact related by ScherfFer : in describing a series 

 of experiments, the view of which was to verify the effect 

 pointed out by BufFon, and in which he looked at the object 

 for a long time, he thus expresses himself (§ 16 of his me- 

 moir) : " When I looked at white spots upon coloured paper, 

 they seemed slightly tinted in the interior of their periphery 

 with the colour of the ground. I would not, however, war- 

 rant that this effect always takes place." It appears then that 

 from a prolonged contemplation there may arise, on the part 

 of the retina occupied by the image of the object, an impres- 

 sion of the colour which surrounds that object. This can be 

 ascertained by the following process, which renders the effect 

 very intense. Place on a sheet of red paper exposed to a 

 clear day-light, a small square of gray paper of such a tint 

 as will appear neither paler nor darker than that of the sheet. 

 Mark a black point in the middle of that small square, and 

 keep vour eyes fixed on this point for a sufficient length of 

 time. You will shortly see on the square a feeble red hue 

 manifest itself, which acquires more and more intenseriess, 

 according as the colour of the sheet appears to darken. Now, 

 when a red object is placed on a white surface, the feeble tint 

 of whiteness which seems to blend itself with the colour of 

 that object, becomes a necessary consequence of the facts 

 above mentioned. It remains to be shown in what manner 

 these facts relate to my theory. 



If accidental colours are owing to a reaction of the retina, 

 it must be necessarily admitted that, during the contemplation 

 of the object, a tendency to the reaction develops itself, that 

 is to say, that the retina opposes to the action of coloured 

 rays a certain progressive resistance ; for we cannot conceive 

 a reaction where there was no resistance. Now, this pro- 

 gressive resistance, or tendency more and more intense to 

 constitute itself in the opposite state from which results the 

 sensation of the accidental colour, must necessarily manifest 

 itself by a gradual falling off in the apparent brightness of 

 the object looked at. Thus, in the first place, is derived from 

 my theory, as a necessary consequence, the fact that an object 



