, On Coloured Shadozas. B^ 



when seen through a tube, which excluded all comparison of one 

 shadow with anotherj^ all these shadows appeared black, he con- 

 cluded from thence that all these effects are mere optical decep-' 

 tions*. M' 



M. de Grotthuss arrived at nearly the same conclusion, but 

 by a different process f. He knew the phenomenon of the blue 

 and yellow shadows, which are produced by the concurrence of 

 the light of a candle and that of the twilight ; he also knew the 

 impression which the long continued observation of coloured 

 plates produces upon the retina; an impression which af- 

 terwards reproduces in the organ spots tinged with colours ex- 

 actly complementary, in the scale of the spectrum, to those on 

 which the eye has been fixed ; and he in like manner considers 

 the phenomenon of coloured shadows as a physiological decep- 

 tion, as the result of the fatigue caused by an effort of the organ 

 in the same direction, and of the disturbance of an equilibrium of 

 sensibility in it. 



After giving this historical narration, Mr Zschokke remarks, 

 that none of the hypotheses explains all the cases in which sha- 

 dows appear coloured, and he proceeds to the exposition of a 

 new theory. Let us first give an account of the fundamental 

 phenomena, the causes of which form the subject of inquiry. 



Coloured shadows are produced in the solar light, when it is 

 refracted by the vapours of the lower strata of the atmosphere, or 

 reflected by the clouds. Thus, l^^, the colouring is percep- 

 tible chiefly at sunrise and sunset, when the sun is not higher 

 than from ten to twenty degrees above the horizon, ^d, In 

 winter, the shadows are sometimes coloured at noon, because at 

 that season in our latitude the sun scarcely rises to the height of 

 twenty degrees. In summer, they are only coloured in full 

 day when the sky is overcast, and the clouds reflect a strongly 

 coloured light. Sd, The more deeply the rays penetrate into 

 the lower strata, the more strongly are the shadows coloured, 



• See Philosophical Transactions 1 794 ; or in the Biblioth. Britann. vol. i. 

 p. 339, an extract of Count Rumford's paper, terminated by a note (p. 372) 

 upon coloured shadows, and the authors who have treated of them. 



f See in Schweigger's Beytrag. zur Chemie und Pbysik, vol. iii. p. 148. an 

 abridgment of M. de Grotthus's Paper on the Accidental Colours of Shadows, 

 and on Newton's Theory of Colours. 



