COLOURED, a curious phenomenon in optics, which was observed by Professor Scherlief of Vienna, and afterwards by Count Rumford, who made the discovery while prosecuting his experiments on light.

"Desirous," says the count, "of comparing the intensity of the light of a clear blue sky by day with that of a common wax candle, I darkened my 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 75° 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 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 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 day-light, was illuminated by the candle, was yellow; while the other, corresponding 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 appearance, which was not only unexpected, but was really in itself the highest degree striking and beautiful, I found upon repeated trials, and after varying the experiment in every way I could think of, to be so perfectly permanent, that it is absolutely impossible to produce two shadows at the same time from the same body, the one answering to a beam of day-light, and the other to the light of a candle or lamp, without those shadows being coloured, the one yellow, and the other blue.

"If the candle be brought nearer to the paper, the blue shadow will become of a deeper hue, and the yellow shadow will gradually grow fainter; but if it be removed farther off, the yellow shade will become of a deeper colour, and the blue shade will become fainter; and the candle remaining stationary in the same place, the same varieties in the strength of the tints of the coloured shadows may be produced merely by opening the window-shutter a little more or less, and rendering the illumination of the paper, by the light from without, stronger or weaker. By either of these means, the coloured shadows may be made to pass through all the gradations of shade, from the deepest to the lightest, and and it is not a little amusing to see shadows thus glowing with all the brilliancy of the purest and most intense prismatic colours, then passing suddenly through all the varieties of shade, preserving in all the most perfect purity of tint, growing stronger and fainter, and vanishing and returning, at command."

The count is clearly of opinion, that the causes of the colours of these shadows arise from the different qualities of the light by which they are illuminated; but he does not think it so evident how they are produced. Perhaps it may be said, however, that all the phenomena of coloured shadows which the count enumerates may be accounted for by the theory of Professor Scherifer.