VOL. LXXXVI.] PHILOSOPHICAL TRANSACTIONS. 7'H 



fact; for any inflammable body whatever, on being lighted, burns at first with a 

 blue or violet flame, and afterwards has its flame of 2 or 3 distinct colours, blue, 

 white, red, &c. as is seen remarkably in the case of a candle. Nay, I have 

 observed in the flame of a blow-pipe all the 7 primary colours at once. When 

 indeed a body is burnt in pure oxygenous gaz, the combination is so rapid, that 

 white light alone is precipitated undecomposed ; but in common air, where the 

 azotic gaz impedes the combustion, the above phenomena are obvious. 



4. A curious phenomenon has often surprized philosophers, namely, blue 

 shadows. These I have observed at all times, when the paper on which I received 

 them was illuminated by the sky, and any other light; and the reason of them I 

 take to be this, that the shadow made by one light is illuminated by the blue 

 rays from the sky ; for I have often observed purple, and even reddish ones, when 

 the sky or clouds happened to be of those colours; and this account of the 

 matter is confirmed by an experiment. Having received the coloured spectrum 

 made by a prism with a large refracting angle, on a sheet of rough white paper, 

 and held above it another sheet, I stopped all the rays that illuminated the first, 

 except the blue, and violet, and red; and if I held a body between the blue and 

 the 2d paper, its shadow was red; and if I held a body between the red and the 

 paper, its shadow was blue; and so of other colours. This I take to amount to 

 a demonstration of the thing.* 



5. Passing over other phenomena of less note, I come now to one that has 

 divided opticians more than any other; I mean the coloured fringes that surround 

 the shadows of bodies. I made several observations on these, which enable me 

 to conclude that each fringe is an image of the luminous body; for holding be- 

 tween my eye and a candle '2 knife blades, as I approached the one to the other, 

 the edge of the candle seemed multiplied, and soon became coloured, coming 

 wholly away from the candle, and as the knives approached still nearer, became 

 distinct dilated images, highly tinctured with the prismatic colours; and just 

 before the knives met, the candle, whose edges had been all along coloured with 

 red and yellow, became much distended, till at last it was divided in the middle, 

 one half seeming to be drawn away by each knife, and then it wholly disappeared. 

 I have observed 3 kinds of these images: 2 without and 1 within the shadow; 

 the first had its colours in the order from the shadow, red outermost, and violet 

 innermost ; the 2d and 3d had the colours in the contrary order, but the 2d was 

 so very faint that I could never perceive it unless when let fall on my eye. All 

 this is easily explained by the different flexibility of the rays. In fig. g, let ad 

 be a body, by which the rays sdt and sVx' pass; and let sd be within ad's 

 sphere of inflection, and s'd' within its sphere of deflection ; then sd will be 



* Since writing the above, I find the same explanation of the matter given by Mr. Melvill, and 

 some of the French academicians, particularly Messieurs Buffon and Beguelin; also Count Rumford; 

 but I have thought fit to keep it in, on accomit of the experiment that occurred to me in illustration 

 of it. — Orig. 



