Dr Brewster on some Affections of the Retina. 289 



indirect impressions, and from the influence of highly attenu- 

 ated light upon the retina, seem to have escaped their notice. 



If we look at a narrow slip of white paper placed upon a 

 black or a coloured ground, it will never appear to vanish, 

 however long and attentively we view it. But if the eye is 

 fixed steadily upon any object within two or three inches of 

 the paper, so as to see it only indirectly, or by oblique vision, 

 the slip of paper will occasionally disappear, as if it had been 

 removed entirely from the ground, the colour of the ground 

 extending itself over the part of the retina occupied by the 

 image of the slip of paper. 



If the object seen indirectly is a black stripe on a white 

 ground, it vanishes in a similar manner ; and, what is still 

 more remarkable, the same phenomena of disappearance take 

 place whether the object is viewed zvith one or with both 

 eyes. 



When the indirect object is luminous, like a candle, it never 

 vanishes entirely, unless it is placed at a great distance ; but 

 it swells and contracts, and is surrounded by a halo of nebu- 

 lous light, so that the excitement must extend itself to conti- 

 guous portions of the retina which are not influenced by the 

 light itself. 



If we place two candles at the distance of about eight or ten 

 feet from the eye, and about twelve inches from each other, 

 and view the one directly and the other indirectly, the indi- 

 rect image will be encircled with a bright ring of yellow light, 

 and the bright light within the ring will have a pale blue co- 

 lour. If the candles are viewed through a prism, the red and 

 green light of the indirect image vanish, and leave only a large 

 mass of yellow, terminated with a portion of blue light. 



While performing this experiment, and looking steadily and 

 directly at one of the prismatic images of the candle, I was 

 surprised to observe that the red and green rays began to dis- 

 appear, leaving only yellow and a small portion of blue ; and 

 when the eye was kept immoveably fixed on the same part of 

 the image, the yellow light became almost pure white, so that 

 the prismatic image was converted into an elongated image of 

 white light. 



VOL. HI. NO. II. OCTOBER 1825. T 



