Appearances from the Contemplatmi of Coloured Objects. 445 



latter sensation being also contrary to the green negative tint 

 which covered the small object at the first instant, there re- 

 sults from thence a new cause, which must concur with the 

 former to weaken this negative tint. Much more, if the cir- 

 cumstances are such that the green negative tint be only faint, 

 as it commonly happens when we look in a clear daylight, at 

 a gray object placed on a red ground ; then the above-named 

 cause increasing continually, it must soon completely neu- 

 tralize the green negative tint, and produce afterwards the 

 apparition of a positive tint more and more intense. Thus is 

 explained in the simplest manner the appearing, on the sur- 

 face of the object, of a tint similar to that of the ground, and 

 increasing in intenseness with the continuance of the contem- 

 plation. When the ground is white, the negative action of 

 the corresponding part of the retina exercises itself at once 

 upon all the rays which compose whiteness ; and consequently 

 in the interior of the space occupied by the image of the 

 small object, the positive action of the organ must produce 

 the sensation of all those rays combined together, that is to say, 

 a sensation of whiteness. A small object placed on a white 

 ground must therefore, by a sufficiently prolonged contem- 

 plation, appear to take a slight tint of whiteness. In the ex- 

 ample before mentioned, I have supposed the object to be 

 black or gray, to show the reasonings in a more simple light : 

 but we conceive that they are also applicable to an object of 

 any colour whatever. 



I shall now quote new experiments which are very simple, 

 and confirm the above explanation, in a manner that seems 

 to me conclusive. They have been repeated with the same 

 success by other persons. 



1. Place on a black surface a sheet of coloured paper, for 

 instance red, and in the middle of that sheet lay a small 

 square of black or gray paper marked with a white point. 

 Fix your sight on this point, till a red tint perfectly visible 

 shows itself on the surface of the small square. Then, re- 

 taining this latter in the same place, withdraw suddenly the 

 coloured sheet, so that the small square thus remains sta- 

 tioned on the black surface. At that vei'y instant, you will 

 see the red tint which covers it acquire a remarkable increase 

 of intensity. In this case, in fact, the surrounding negative 

 reaction of the retina operating freely, the positive reaction 

 corresponding to the small square must also operate with 

 freedom, and thus become much more intense. 



2. Instead of withdrawing as above, the sheet of red paper, 

 cast your eyes rapidly on the ceiling of the apartment, or 

 cover them completely ; then you will sec a surface of an in- 



