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X. An Account of some Experiments upon coloured Shadows . 
J3y Lieutenant-General Sir Benjamin Thompson, Count of 
Rumford, F. R. S . In a Letter to Sir Joseph Banks, Bart , 
P. R. & 
Read February 20, 1794. 
DEAR SIR, 
Since my last letter, being employed in the prosecution of 
my experiments upon light, I was struck with a very beau- 
tiful, and what to me appeared to be a new appearance. De- 
sirous of comparing the intensity of the light of a clear sky, by 
day, with that of a common wax candle, I darkened my room, 
and letting the daylight from the north, coming through a 
hole near the top of the window-shutter, fall at an angle of 
about 70 s upon a sheet of very fine white paper, I placed a 
burning wax candle in such a position that its rays fell upon 
the same paper, and as near as I could guess, in the line of re- 
flection of the rays of daylight from without ; when inter- 
posing a cylinder of wood, about half an inch in diameter, be- 
fore the centre of the paper, and at the distance of about two 
inches from its surface, I was much surprised to find that the 
two shadows projected by the cylinder upon the paper, instead 
of being merely shades without colour, as I expected, the one 
of them, that which, corresponding with the beam of daylight, 
was illuminated by the candle, was yellow ; while the other, 
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