MEMOIR ON COLOURED SHADOWS. 



285 



lamp was five decimetres from the enlightened plane, the 

 fliadow caufed by an opaque black body was blue. On bring- 

 ing the lamp nearer the colour of the ihadow changed fuccef- 

 iively; from blue it changed to indigo, from indigo to violet, 

 and the violet blackened gradually. When the lamp was very 

 clofe, the fliadow was of a violet black colour, exa6lly fimilar 

 to that produced by the light of the fun on a clear fummer's day. 



This experiment fucceeds very well on days when the fun is 

 hid by clouds, all bodies are then illuminated by the light of 

 the fky. 



A fimilar refult may be obtained by the inverfe method, wkh an artifi. 

 tiiatistofay, by illuminating the furface with an artificial f|^j.|igj'^'^^hi^'» 

 light of conflant intenfity, and fucceffively increafing the in- 

 tenfity of the light of the (ky. 



If in a dull morning, before the appearance of twilight, a 

 white furface is illuminated by the light of a lamp placed at 

 five decimetres difiant from the enlightened furface, the 

 fliadow of an opaque black body placed at a fmall diftance 

 from the furface, is black very flightly violaceous. As foon 

 as the twilight appears, the tint changes, the intenfity of the 

 violet increafes. As the day brightens, the violet of the fliadow 

 effaces the black tint ; at length the fliadow becomes violet, 

 indigo, and Is blue when the light of the day is completely 

 developed. 



Repeated experiments with a taper or a candle, have 

 given the fame refults. 



It follows, from the obfervations detailed, that every General refu't, 

 fliadow, formed on a body illuminated, at the fame time, by 

 the light of the atmofpherc and the direct light of the fun ; or 

 by the light of the fky and an artificial light, fuch as a lamp, 

 a taper, a candle, is coloured in all cafes wherein the light 

 of the fun, or the artificial light is intercepted by an opaque 

 black body ; and that the colour of the fliadow varies from 

 green to violet black, in the ratio of the Intenfity of the light 

 of the fun, or the artificial light, compared with that of the 

 atmofphere. But on what depends the colour of the at- 

 mofphere ? what caufes that variation of colour in the ratio 

 of the comparative intenfity i This is what we ihall examine 

 in another Memoir. 



(To be concluded in our next.) 



SCIENTIFIC 



