282 MEMOIR ON COLOURED SHADOWS, 



XVII. 



A Firjl Memoir on colnured Shadozvs *'. By Cit. 

 J. H. Kassenpratz. 



Shadows are ge- VV E are accuftomed to confider as black, the fliadow 



neouny counted •^'^'^'"^^ by an opaque body, which intercepts the hght falling 



^lack. on a wiiite pafleboard, although in reality, nothing is more 



difficult to obtain than black fhadows, becaufe it appears, that 



to procure a black fiiadow, the light muft abfolutely be a 



point, and no fort of reflecled light muft reach the illuminated 



furface. 



Phenomena of On a furface illuminated by a light which ha§ magnitude, 

 Aijdows. ,ni-. -.7 , ,, 



the inadow is always accompanied by a coloured penumbra. 



Whenever the illuminating body is at a greater diftaqce thap 

 a metre from the body enliglitened^ and that which intercepts 

 the light, is nearly five decimetres from this body, the pe- 

 numbra and fliadow have diftin^ colours, which are in a 

 great meafure dependent upon the nature of the combuftible 

 body which yields the light. 

 The blue iha- Since the period when Leonardo da Vinci noticed the blue 



dow at fun-rile . , .... .... . . .~ 



and fiin-fet, is Hiadow vvliich IS perceived in the morning at tun-nle, and in 



notdifFerent ^he evening at fun-fet, this Ibadow has been attended to, and 

 lawsof fhadows^. "^'"'"^''^'^^ ^^^^ endeavoured to explain its produdion, as 

 forming a phenomenon peculiar and diftin6l from thofe of 

 other ftiadows. Neverthelefs, this fliadow has no particu- 

 larity ; all the fliadows which are examined are coloured, 

 even thofe produced by the noon-tide fqn of a clear fummer's 

 day, which are commonly called black Jhadows. The cp. 

 loured fliadows are produced by two or more diftindt lights 

 or by the feparated parts pf the fanie light which a6l dif- 

 ferently. 

 Variety ofco- Nothing is more diverfified than the colours of fliadows; on 

 noticing them with attention, we remark among them all 

 the prifmatic colours : we diftinguifh red, orange, yellow, 

 green, blue, violet fliadows, more or lefs mixed with 

 black. 

 Thefubjeais The objecl of this firft Memoir, is to make known the 

 heyo'nrthu'^ g'"^^^ variety of coloured fliadows which may bgdiftinguifl^ec^ 



loured fhadows. 



Memoir, 



From Journal de L'EcolePolytechnique, Tom. IV. p. 272. 



and 



