374 PHILOSOPHICAL TRANSACTIONS. [anNO 1794. 



But he afterwards found means to demonstrate the very great transparency of 

 flame by a still more simple experiment. Suspecting that the only reason why 

 bodies are not visible through a sheet of vivid flame is, that the light of the 

 flame affects the eye in such a manner as to render it insensible to the weaker 

 light emitted by, or reflected from the objects placed behind it, he conceived 

 that a very strong light would not only be visible through a weak flame, but 

 also, as all transparent bodies are invisible, that it might perhaps cause the flame 

 totally to disappear; to determine that fact, he took a lighted candle at mid-day, 

 the sun shining moderately bright, and holding it up between his eye and the sun, 

 he found the flame of the candle to disappear entirely. It was not even necessary, 

 in order to cause the flame to become invisible, to bring it to be directly between 

 the eye and the body of the sun ; it was sufficient for that purpose to bring it 

 into the neighbourhood of the sun, where the light was very strong: even in a 

 situation in which the light was not so strong as to dazzle the eye so much as to 

 prevent its seeing very distinctly the body of the candle and the wick, not the 

 least appearance of flame was discernible, though the candle actually burnt the 

 whole time very vigorously, 



X. An Account of some Experiments on Coloured Shadows. By Lieutenant- 

 General Sir Benjamin Thompson, Count of Rumford, F. R, S. p. 107. 

 Since the foregoing letter, being employed in the prosecution of his experi- 

 ments on light. Count R. was struck with a very beautiful, and to him new ap- 

 pearance. Desirous of comparing the intensity of the light of a clear sky by 

 day, with that of a common wax candle, he darkened the room, and letting the 

 day light from the north, coming through a hole near the top of the window- 

 shutter, fall at an angle of about 70° on a sheet of very fine white paper, he 

 placed a burning wax candle in such a position that its rays fell on the same 

 paper, and as near as he could guess in the line of reflection of the rays of day 

 light from without; when interposing a cylinder of wood, about half an inch in 

 diameter, before the centre of the paper, and at the distance of about 2 inches 

 from its surface, he was much surprizeu to find that the 2 shadows projected by 

 the cylinder on the paper, instead of being merely shades without colour, as he 

 expected, the one of them, that which, corresponding with the beam of day 

 light, was illuminated by the candle, was yellow; while the other, correspond- 

 ing to the light of the candle, and consequently illuminated by the light of the 

 heavens, was of the most beautiful blue that it is possible to imagine. This ap- 

 pearance, which was not only unexpected, but was really in itself in the highest 

 degree striking and beautiful, he found, on repeated trials, and after varying the 

 experiment in every way he could think of, to be so perfectly permanent, that it 

 is absolutely impossible to produce 2 shadows at the same time from the same 



