Appearances from the Contemplation of Coloured Objects. 339 



your sight towards the same point, without winking the eye; 

 you will see nothing but darkness at the beginning of this 

 operation, after that, towards the point you look at, there 

 will appear a mnch greater obscurity than that of the remain- 

 der of the apartment. Continue thus to keep your sight 

 steadily fixed ; gradually in the middle of this obscurity, a 

 reddish colour will manifest itself; it "will become insensibly 

 enlivened^ its brightness will increase, and at last it 'will ac~ 

 quire the colour of the fame" 



Thus, in this experiment, the obscure negative image, that 

 is to say, contrary to the brilliant impression of the flame, 

 was gradually changed into a new luminous image having the 

 colour of this flame ; that is, into a positive image. I must 

 here remark, that Rozier recommends tofx the sight towards 

 the savie point "without •winJcing the eye, and further, to keep the 

 sight steadily fxed, so that these precautions remove every 

 supposition of pressure or distortion of the eyes, which could 

 alone, as the anonymous author maintains, cause the image 

 to vary. I shall add, that this experiment has not been made 

 with the view of giving weight to any theory ; Rozier presents 

 it simply as a fact that he does not seek to explain. With re- 

 spect to more multiplied alternations of the two impressions, 

 alternations which the anonymous author has not been more 

 successful in verifying, unluckily no one, as far as I know, 

 had noticed them before me. For which reason I have taken 

 care to associate other persons with myself, who have experi- 

 enced the same effects, and I have cited among them M. 

 Quetelet, a name which ought to inspire the fullest confidence 

 (see my article oi Annales, page 392). 1 will here relate the 

 experiment such as I describe it in that article, pages 393 

 and 394'. 



" I applied to one of m}' eyes a black tube about 50 centi- 

 metres long by 3 in diameter, at the same time covering the 

 other eye completely with a handkerchief, and I looked stead- 

 fastly, during a minute at least, on a red paper exposed to a 

 clear day-light; after which, withdrawing the tube suddenly 

 without uncovering the other eye, I looked at the white ceil- 

 ing of the apartment. I then saw a green circular image, 

 which, after some time, gave place to a red one of a feeble 

 intensity indeed, and of a very short duration, but perfectly 

 visible; afterwards, the green colour reappeared, which, soon 

 after, was again succeeded by a reddish image, and thus three 

 or four limes successively, the two opposite impressions being 

 less and less intense." 



I shall only subjoin that the coloured paper ought to be 

 suflicicnlly large for its edges not to be perceived. In this 

 Z2 



