176 P. C. on the Colours that enter into the [Sept. 



recovers its power to perceive weaker light, the black image 

 becomes lighter, and at length assumes a gray appearance, 

 such as might be expected from a small quantity of white 

 light reflected from a black ground. The change from 

 black to gray corresponds in its circumstances with the 

 gradual appearance of objects in a dark room, after the eyes 

 have been exposed to strong light. 



But, unless the eye be steadily directed to the object, 

 the image, instead of becoming gray, assumes the appear- 

 ance of the accidental colour, of the colour by which it is 

 surrounded. If we look steadily at the image, so as to 

 preserve the gray appearance of it, the accidental colour 

 will in a short time be seen upon its margin, from the 

 involuntary motion of the eyes, precisely as it is observed in 

 looking at a white object upon a coloured ground ; and, the 

 slightest motion of the eyes, which, with the exception of 

 the accidental image, are wholly impressed with the colour 

 of the ground, extends it until the colour becomes uniform, 

 which, indeed, it is not easy, for any length of time, to 

 prevent. 



If we look from one image to the other, which we naturally 

 do when we make the experiment without any particular 

 caution, the accidental colour is much finer than when the 

 eye is impressed with the coloured ground only ; the coloured 

 image being much more vivid than the ground upon which 

 it is seen. 



In order to satisfy myself of the correctness of this ex- 

 planation, I made such an arrangement that the whole of 

 the coloured glass, with the exception of the two images of 

 my finger, which formed the shadow, was covered ; so that, 

 upon looking at the two images in succession, the accidental 

 image fell upon a part of the eye which was impressed with 

 the coloured image, and upon a part of it not thus impressed, 

 alternately ; in the former case, it assumed the accidental 

 colour, and in the latter, the gray appearance I have before 

 described : the result of the experiment was very decided, 

 as, by a slow motion of the eye, both colours were seen at 

 the same time. The alternations ought not to be too rapid. 



When the coloured glass is tilted considerably, a breadth 

 of white light is reflected by the mirror to the under side of 

 the coloured glass, which meets the eye after one transmis- 



