USEFUL PROJECTS* 



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sinip, and open one of the window- 

 shutters a little, about half or three 

 qunrters of an incli for instance j 

 when the candle being placed upon 

 a table or stand, or given to nii 

 assistant to hold, in such a situation 

 that the rays from the candle may 

 "meet those of day-light from witl:- 

 out, at an angle of about 40°, at 

 the surface cff a sheet of white pa- 

 per, held in a proper position to re- 

 ceive them, any solid opaque body, 

 a cylinder, or even a Hnger, held 

 before the paper, at the distance of 

 two or three inches, will project 

 two shadows upon the paper, the 

 one blue, and the other yellow. 



If the candle be brought nearei- 

 to the paper, the blue shador^' will 

 become of a deeper hue, and the 

 yellow shadow will o-raduallv s^row 

 famter; but it it be removed farther 

 ofl^", the yellow shadow will become 

 of a deeper colour, and the 

 blue shadow will become faint- 

 er ; and the candle remaining 

 stationary in the sams place, t';:e 

 -same varieties in the strength of tiie 

 lints of the coloured shadows may 

 be produced merely by opening ti;e 

 v/indow-shuttera little more or less 

 and rendering the illumination of 

 the ])aper by the light from without 

 stronger or weaker. Bv either of 

 these means, the coloured shadows 

 may be iriade to pass ihrougii all the 

 ^gradations of shade, Iroin ihe deep- 

 est to the lightest, and vice versa ; 

 and it is not a iittic amusing lo see 

 shadows, thus glowing wiih all the 

 brilliancy of the purest and most in- 

 tense prismatic colours, then passing 

 suddenly through all the varieties of 

 shade, piescr\ ing in ail tl.e most per- 

 fect purity ol tint growing stronger 

 and fainter, and vanish !;g and re- 

 turning at command. 



With respect to the causes of t!ie 

 colours ui these shadowd, there is 



no doubt but they arise from ihedif- 

 ferent qunjites of ! he light by which 

 they are liluruinated; but how they 

 are produced, does not appear to 

 lue so evident. That the shadow 

 corresponding to the beam of day- 

 light, which is illuminated by the 

 yellow light of a candle, should be 

 of a yellowish hue, is not surpising : 

 but why is the shadow correspond- 

 ing to the light of the candle, and 

 which is illuminated by no other 

 light tlian the apparently white light 

 of th« heaveirs, l-luc? I at first 

 thought that it migiit arise from the 

 blueness of the sky; but finding that 

 the broad daV-light, reflected from 

 the roof a neighbouring house co- 

 hered wi1h the whitest new fallen 

 snow, produced the same blite co- 

 lour, and, if possiple, of a still more 

 beauriful x'uit, I was obliged to 

 abandon that opinion. 



lo ascertain with somedegree of 

 precision the real colour of the light 

 emitted by a candle, I placed a 

 ligiited wax candle, well trimmed, 

 in the open air, at mid-day. at a 

 time v>htn the g;"ound was deeply- 

 covered with r.ew fallen snow, and 

 the heavens were overspread with 

 "liite clouds; when the flame of 

 tiie candle, fr.r from !ieing white, as 

 it appears to be wi:on viewed bv 

 right, was evidently of a very de- 

 cided yellow colour, not even ap- 

 proaching to wliiiness. The flame 

 of" an Argand's inuip, exposedatthe 

 same time in the open air, appeared 

 to be of tJie same yellow hue. But 

 liiemostsirikingmanner of shewing 

 the yellow huf of the light emitted 

 by lamps and candles, is by ex- 

 posing them in the direct rays of a 

 bright meridian sun. In that situa- 

 tion the flame of an Argand's lamp, 

 burning witli its greatest brilliancy, 

 appears in the form of a dead yel- 

 low semi-lianspa'rcnt snioke, Plow 



Z 4 trail- 



