i)ti Coloured Shado'Ws. ' {-4^1 



light, is black ; if only one of the coloured rays of which it is 

 <;omposed is intercepted, the part cannot produce the same effect 

 as the whole ; the coloured ray cannot therefore project a black 

 shadow ; this shadow must itself be coloured.*" 



Now, what will be the colour of the shadow projected by a 

 ray of a given colour ? To find this, M. Zschokke made the 

 solar rays pass through disks of glass variously coloured ; and 

 receiving the light by this procedure upon a white surface, he 

 presented before this surface an opaque body, in order to form 

 shadows with it. He took care to make the experiment when 

 the sun was at a great height upon the horizon, to prevent any 

 natural colouring of the shadows mingling with that which he 

 produced artificially. He then found, that, in the 



lied rays, the shadow is pale blue. 



Orange, a little deeper blue. 



Yellow, a violet blue. 



Green, a purple violet. 



Pale blue, red. 



Deep blue, orange. 



Violet, green. 



and that thus to each colour of the ray there corresponds, in the 

 shadow which it projects, a colour which would itself project a 

 shadow of the same tint as the ray. 



Such is Mr Zschokke''s theory in brief; we regret that we 

 cannot follow him in developments from which his style, always 

 animated and descriptive, takes away the dryness of a scientific 

 dissertation. 



" The hypothesis of Mr Zschokke,'"" says Mr Trechsel, " re- 

 commends itself at first sight by its precision, and, if one may so 

 speak, by its paradoxical nature. One fancies he sees in it the 

 great law of polarity, which appears to manifest itself in almost all 

 the branches of natural philosophy. Besides, the most import- 

 ant discoveries have been in fact but gleams of light emitted 

 by geniuses superior to their age, hypotheses imagined a priori., 

 which have been recognised as true by observations and re- 

 searches made afterwards." These considerations, which ought 

 to recommend the hypothesis in question to the attention of na- 

 tural philosophers, have engaged Mr Trechsel the younger, to 

 repeat with his father the experiments of Mr Zschokke, and to add 



