OQ MEMOIR ON COLOURED SHADOW*. 



Part the Third. 



Shadows from IF in a clear moon-light night, at a time when the lamps 



different colours. ^^^ alight in Paris, an opaque black body be brought near a 



white paper, enlightened both by the moon and a lamp, two 



diftind coloured lliadows will be perceived, one reddifli, the 



oiher bluifh. 



If a taper, a candle, a lamp, or any other light, be brought 

 near a white pafteboard enlightened by the moon, an opaqup 

 body placed at a fmall diftance from the pafteboard, forms two 

 coloured ftiadows ; that occalioned by intercepting the light of 

 the moon is reddifti, and that produced by the other light is 

 blue. 



Thefe two ftiadows may be obtained by enlightening a fur- 

 face with the atmofpheric light and that of a lamp ; but this 

 requires that the light of the atmofphere muft enter the r,oom 

 where the experiment is made, through a fmall opening, in 

 order that the fliadow may be well defined when this light is 

 fingle. 

 Theeffeawas By admitting the light of the atmofphere into a darkened 



not varied by room through an Opening of a decimetre(four inches) in diameter, 

 the nature of one , .„ .^ . ,. n i i i i • i- i , , 



combuftible and illummatmg a white palteboard by this light and that of a 



body made ufc lamp, we have found that when the pafteboard was one or two 

 * metres (yards) from the opening, two coloured ftiadows were 



produced; that of the atmofphere is conftantly red, and that 

 of the artificial light, blue. We have employed the lights 

 from coals, wood, tapers, candles, alcohol, and even hidro- 

 gen gas. 



The greateft part of the ftiadows obtained from two different 

 lights are of two tints, the one reddifti, the other bluifti. 



The variations in (lie colour of ftiadows are independent of 

 the intenfity of the lights which illuminate the ftirface. We 

 were careful in all the experiments we are going to relate, to 

 place the light at fuch diftances that they fliould enlighten 

 equally that part of the white pafteboard on which the ftiadows 

 were projeded ; and to this end we followed the method made 

 ufe of by Bouguer, and which confifts in receiving through 

 two holes, on a piece of oiled paper, the rays from two dif- 

 ferent lights, and removing the ftrongeft, or bringing forward 

 the weakeft, until the two lights were of equal intenfity ; 

 .always being careful to place the eye at an equal diftance from 

 the two enlightened points. 



The 



