On Coloured Shadoivs. 33 



valuable performances, has charged himself with this task in a 

 memoir which he has latterly communicated to us. 



The subject is delicate and contestible. We shall give 

 in succession the explanations of the phenomenon in question 

 given by the two authors, announcing at the same time that we 

 do no hesitate to adopt, at least in its fundamental parts, the 

 opinion of Mr Trechsel. 



M. Zschokke, at the beginning of his memoir, gives an ac- 

 count of the authors who have observed coloured shadows, and 

 attempted to explain them. It will not be uninteresting to go 

 over this ground. Shadows coloured in blue are those which have 

 been most frequently remarked, because in fact nature presents 

 them oftenest to us. Priestley, in his History of Optics, states 

 that this phenomenon was for the first time observed and de- 

 scribed, about the middle of the seventeenth century, by Otto 

 Guerick, the celebrated inventor of the air-pump ; but he is 

 wrong, for Leonardo da Vinci speaks of it in hi^ Treatise on 

 Painting, written in the fifteenth century. This able artist 

 sought to discover, with all the interest excited by a subject of 

 so much importance to his art, to what was owing the colouring 

 of shadows in blue. He only saw in it a reflection of the colour 

 of the sky, or rather of the atmosphere, having recourse for this 

 phenomenon to the same explanation as for the purple tints, 

 which colour rocks and buildings, before the rising and after the 

 setting of the sun, or for the greenish reflection which diffuses 

 itself upon the sides of a vessel, or upon the piles of a bridge 

 above a deep and limpid body of water. Bouguer, in his Optics 

 (1729), Buffon, in his Memoirs of the Academie des Sciences 

 for 1743 ; Begnelin in those of the Berlin Academy for 1767"; 

 Monge in 1789, and other natural philosophers, have more or 

 less adopted the opinion of the celebrated painter. 



Buffon had the merit of contributing powerfully to direct the 

 attention of observers toward the coloured shadows that form in 

 the solar light. " I observed,'' says he, " during the summer 

 of the year 1743, more than thirty sun risings and as many set- 

 tings. All the shadows that fell upon white, as upon a white 

 wall, were sometimes green, but most commonly blue, and of a 

 blue, as lively as the most beautiful azure, I shewed the phe- 

 nomenon to several persons, who were as much surprised as my- 



OCTOBER DECEMBER 1826» C 



