VOL. XXIX.] I'HILOSOPHICAL TRANSACTIONS. 241 



it must be nearer to one eye than to the other, except it be just in a line from 

 the nose between the two eyes straight forward. 



Hence it is, that if there be two candles set before any one, the first at the 

 distance of one foot, and the second at the distance of 2 feet, from the eyes ; 

 he that looks at the second candle at b will see it single, but see the first candle 

 or the candle a double ; one appearance being in the line ADy, the other in 

 OAE, because it paints itself on oo in the retina of each eye, which points are 

 not the middle points, but farther from the nose than the middles mm. So if 

 B be the first candle, and c the second, he that looks at b will see c double, 

 because it is painted in the retina at the points nn nearer the nose than mm ; 

 and so will appear to be in the same position as pr, in fig. 27. 



If y^ be two candles so disposed, fig. 30, that by the interposition of a per- 

 forated board pp, y can paint itself only in the eye r, and ^ in the eye l. On 

 making the optic axes meet at b, and to tend towards ^ and y, ^ and y will each 

 paint an image on the middle of the retina of each eye, by crossing their rays 

 at b: and thus the two candles will appear to be but one, or rather to be in one 

 place, on account of the communication of the middle of each retina. But if, 

 instead of the candles, ^ be a piece of red silk, and y a piece of green silk, the 

 same position of the eyes will make an image at b, appearing like a red and 

 green spot together, without a mixture of the colours. If ^ be a red hot iron, 

 and y a candle of sulphur, the phenomenon will be more distinct. If the optic 

 axes be turned directly towards y and ^, as if there was no board fp in the way, 

 there will appear two holes in the board, the one having the red hot iron in it, 

 the other the candle. 



Now if, of the refracted rays of the candle in the first case, fig. 25, those 

 which diverge from each other, so as to fall into each eye, cause the same 

 sensations respectively, as the rays which come from a red-hot iron and those 

 which come from a blue candle ; it is evident that the candle in the first case 

 affords red-making and blue-making rays after refraction, and that those rays 

 are differently refrangible ; the red bR, fig. 25, the least refrangible, as declin- 

 ing less from the perpendicular tttt ; and the purple as bp declining most from 

 the said perpendicular. 



The same will, cseteris paribus, be found true in the intermediate rays ; and 

 to be certain that the experiment is as I have related it, the planes af and fd of 

 the bar may be covered with paper. 



VOL. VI. I I 



