MEMOIR ON COLOURED SHADOWS. 2g3 



and which we have obferved, and alfo the circumftaix^es which 

 give rife to them. Other memoirs which we purpole to pre- 

 fent after this, if the inftitute fliall think the fubject worthy 

 of its attention, will contain the feries of obiervations we 

 have made to afcertain thecaufes which produce the coloration 

 of fhadovvs in a great number of cafes. 



We fhaii divide this Memoir into three parts : the firft will Divifionof the i 

 have for its obje6l the colour of the fhadows produced by the J^^" 

 reunion of the light of the atmofphere, and that of the fun, 

 or of the light of the atmofphere and an artificial light, but in 

 ihofe circumllances wherein only a fingle fliadow is percep- 

 tible : the fecond will contain a defcription of the coloured 

 fhadows produced by the light of the atmofphere combined 

 with refleded lights, and fometimes with the direct light of 

 the fun: finally, the third will elucidate the coloured fhadows 

 formed on a body illuminated by two lights, natural and ar^ 

 tificial. 



Part the Firfi. 



If, near a while furface enlightened by the fun and the Obfervations of 

 light of the atmofphere, a black body is placed which '"^^r- J^^^^^^^^^'j'jB' 

 cepts the folar rays, there will be feen on the plane a fhadow 

 which varies from a greenifh blue to a violet black, con- 

 neding through the blue and the violet. The colour of the 

 fliadow depends on the fiate of the atmofphere, the latitude 

 of the place, the meridional and northern declination of the 

 iun, and the time elapfed between its rifing and its paflage to 

 the meridian, and from the latter to its fetting. 



When the fky is clear, the colour of the Ihadow at fun-rife, as produced by 

 at Paris, varies between blue with a flight tinge of green to ''^^"Seo^feafan, 

 violet blue. The firft days of Nivofe, the ftiadow is greenifli- 

 blue; the firft days of Germinal, blue; the firft days ofMef- 

 fidor, indigo with a violet tinge ; the firft days of Vende- 

 miaire, the fhadow becomes again blue to return to a greenifti 

 blue the firft of Nivofe, 



If on a clear day, when the fun is on the equator, the va- as produced by 



nation of th^ colour of the fliadow, if noticed from the in- f^'ji^'^'^tb"^"^ 



■ftant of fun-rife to its pafTage over the meridian, it will be ob- fun° 



ferved that this colour is blue at fun-rife ; that at each eleva- 



ition of the luminary above the horizon, the blue changes; 



that it becomes indigo, violet ; that in the end it blackens, 



and that when the fun is on the meridian, the ftiadow is of a 



JDlackiJh violeto 



'•'- • Ever J 



