On Coloured Shadows. 41 



may be projected from the same opaque body, and the shadow 

 formed in the light of one of the candles be lighted by that of 

 the other. These shadows, as is known, will both be yellowish. 

 Let one of the lights now be coloured red, by making it pass 

 through a plate of glass of that colour ; the shadow coming from 

 the interception of the other light, will immediately assume a 

 red tint, (objective colouring) ; but, at the very instant, the 

 other shadow, which is only shone upon by the pure light of the 

 other candle, will become green, (subjective colouring, pro- 

 duced in the organ of the observer itself, from the defect of the 

 perception of the red ray) ; and vice versa, if one of the sha- 

 dows is objectively and directly coloured green, the other will 

 be subjectively coloured red. 



Mr Trechsel here remarks, that the phenomenon observed by 

 several members of the Helvetic Society of Natural Science, in 

 a chapel near Soleure, appears to be of an analogous nature to 

 the subjective coloration of shadows, in particular to the pheno- 

 menon last cited, and to that of the colouring of the bottom of 

 the camera obscura by the light of the day. It will not be use- 

 less to recall here the description of this phenomenon, such as 

 we have already given it in our account of the tenth session of 

 the Helvetic Society *. All the panes of the windows of the 

 chapel, without exception, are of pale yellow glass ; the frames 

 of these windows, which are of iron, are perforated here and 

 there with small holes of about a line in diameter ; the light 

 which penetrates by these holes, is of the most beautiful blue, 

 even when through them the view is carried upon perfectly 

 white clouds. The same effect is also produced when one of 

 the windows is opened, and the slit thus formed is blue until 

 the opening attains a certain width. We had explained this 

 phenomenon, as probably arising from the psychological effect of 

 contrast. Now this effect, which in general cannot be contested, 

 may be owing to the momentary paralysation in the organ of 

 the faculty of perceiving one of the partial sensations which 

 compose the total impression. 



Mr Trechsel concludes his memoir with the following brief 

 review : 



• See Bibl. Univers. torn. xxix. p. 326. 



