the Inflection , Reflection , and Colours of Light. 255 
on being lighted, burns at first with a blue or violet flame, 
and afterwards has its flame of two or three 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 seven 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 com- 
mon air, where the azotic gaz impedes the combustion, the 
above phenomena are obvious. 
4. A curious phaenomenon has often surprised 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 illumi- 
nated by the blue rays from the sky ; for I have often ob- 
served 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 second 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 demon- 
stration of the thing.* 
* 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 account 
of the experiment that occurred to me in illustration of it. 
