34 On Coloured Shadows, 



self. Difference of season has no effect upon it, for there are 

 not eight hours (15th November 1743) since I have seen blue 

 shadows ; and whoever will give himself the trouble of looking 

 to the shadow of liis fingers at sun-rise and sun-set, upon a bit 

 of white paper, will see like me this blue shadow," &c. The 

 illustrious naturalist also cites a letter of the Abbe Millot, in 

 which he announces to him that at noon, with a cloudy sky, in 

 which some openings were seen here and there in the clouds, he 

 had observed shadows of a beautiful blue upon white paper ; 

 and further, that, under certain particular circumstances, he had 

 remarked green, violet, or yellowish shadows, or shadows sur- 

 rounded with a coloured margin of these different tints. Buffon, 

 recapitulating these various observations made in 1743, adds in 

 1773 : " This blue colour of shadows is nothing else than the 

 colour of the air itself *."" 



M. de Schrank, in the Memoirs of the Academy of Munich 

 for 1812, brought forward again the opinion proposed in 1783 

 by Opoix, a French naturalist little known, supporting it by new 

 arguments. The blue shadows, according to him, come from 

 the inflection of the rays'* tangent to the edges of the solid, from 

 which the shadow proceeds. As the blue rays are very refran- 

 gible, they are more strongly attracted than the others by bo- 

 dies, and thus come to colour the interior of their shadows. O- 

 poix, as well as M. de Schrank, knew well that the violet rays 

 are more refrangible than the blue rays; and they reply to the 

 objection which arises from this circumstance, the one that, in 

 the shadows of thin bodies, the violet rays are sufficiently de- 

 flected to pass beyond the opposite edge of the shadow, and en- 

 ter into the open light ; the other, that, in the case where the 

 body has a sufficient breadth to prevent the application of such 

 an explanation, the rays fall, it is true, into the interior of the 

 shadow, but that the tint which they carry there is too obscure 

 to be perceived. 



Rumford observed not only the coloured shadows formed in 

 the pure solar light, but also the various shadows resulting from 

 several sorts of coloured lights combined with each other and 

 with the solar light ; and thinking that he had remarked that, 



• BufFon, Hist. Nat. d'Min., Memoire viii. 



