17. When the sun has been near setting on a summer evening, I have often observed most beautiful blue shadows upon a white marble chimney- piece. In this case, the weak white light of the evening, which illumines the shaded part of the marble, is to be considered as compounded of two colours, orange and blue. The direct orange rays of the sun at this time, render the orange part invisible, and leave the blue in perfection. 18. And in the same way is to be explained that beautiful and easy experiment mentioned by Count Rumford, p. 103, Phil. Trans. 1794, where a burning candle in the day-time produces two shadows, and one of them of a most beautiful blue colour. The experiment is the more valuable, as it may be made at any time of the day with a burning candle. Almost darken a room, and then by means of a lighted candle and a little daylight, produce two shadows of any small object, as of a pencil, &c., one from the candle, and another from the daylight received at a small opening of one of the window-shutters; the light of the candle will appear orange-coloured in the day-time, and so will that shadow of the body which belongs to, or is made by, the daylight; but the shadow of the body made by the candle, will surprise any person, by being of a fine blue.
