TOL. LXXXIV.] PHILOSOPHICAL TRANSACTIONS. 375 



body, the one answering to a beam of day light, and the other to the light of a 

 candle or lamp, without these 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 re- 

 moved farther off, the yellow shadow will become of a deeper colour, and the 

 blue shadow 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 vice 

 versa; 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 sud- 

 denly through all the varieties of shade, preserving in all the most perfect purity 

 of tint, becoming stronger and fainter, and vanishing and returning at command. 



With respect to the causes of the colours of these shadows, there is no doubt 

 but they arise from the different qualities of the light by which they are illumi- 

 nated; but how they are produced, does not appear so evident. That the shadow 

 corresponding to the beam of day light, which is illuminated by the yellow light 

 of a candle, should be of a yellowish hue, is not surprising; but why is the 

 shadow corresponding to the light of the candle, and which is illuminated by no 

 other light than the apparently white light of the heavens, blue? I at first 

 thought, says Count R. that it might arise from the blueness of the sky; but 

 finding that the broad day light, reflected from the roof of a neighbouring house 

 covered with the whitest new fallen snow, produced the same blue colour, and 

 if possible of a still more beautiful tint, I was obliged to abandon that opinion. 



To ascertain with some degree of precision the real colour of the light emitted 

 by a candle, I placed a lighted wax candle, well trimmed, in the open air, at 

 mid-day, at a time when the ground was deeply covered with new fallen snow, 

 and the heavens were overspread with white clouds; when the flame of the 

 candle, far from being white, as it appears to be when viewed by night, was 

 evidently of a very decided yellow colour, not even approaching to whiteness. 

 The flame of an Argand's lamp, exposed at the same time in the open air, ap- 

 peared to be of the same yellov/ hue. But the most striking manner of showing 

 the yellow hue of the light emitted by lamps and candles, is by exposing them 

 in the direct rays of a bright meridian sun. In that situation the flame of an 

 Argand's lamp, burning with its greatest brilliancy, appears in the form of a 

 dead yellow semi-transparent smoke. How transcendently pure and inconceiv- 



