442 Mr. P. Cooper on Accidental Colours, 



with black paper, against the centre of one of the broad 

 shadows, while an assistant repeatedly interposed a sheet 

 of yellow glass before the lamp whose light corresponded 

 to the shadow he was looking at through the tube; when, 

 ** so far from being able to observe any change in the 

 shadow upon which his eye was fixed, he was not able even 

 to tell when the yellow glass was before the lamp, and 

 when it was not ; and though the assistant, often exclaimed 

 at the striking brilliancy and beauty of the blue colour of 

 the very shadow he was observing, he could not discover 

 in it, the least appearance of any colour at all. But, as 

 soon as he removed his eye from the tube, and contem- 

 plated the shadow with all its neighbouring accompani- 

 ments, the other shadow rendered really yellow by the 

 effect of the yellow glass, and the white paper, which had 

 likewise from the same cause acquired a yellowish hue, the 

 shadow in question appeared to him, as it did to his assis- 

 tant, of a beautiful blue colour." The standard to which 

 both shadows are referred in this experiment, is the mixed 

 light from both lamps, one coloured and the other white. 



M. Hassenfratz discovered by numerous experiments, 

 that the light reflected by the atmosphere, and the direct 

 light of the sun is always different in colour ; and that the 

 shadows produced by the separation of these two kinds of 

 light, vary with the state of the atmosphere, the latitude 

 of the place, the season of the year, and the hour of the 

 day. He noticed that in coloured rooms, *' when several 

 lights direct or reflected concur with the light of the atmo- 

 sphere to enlighten the plane on which the shadow is 

 observed," the number of shadows is almost always two or 

 three, sometimes four or five, and it has even happened, 

 that six were distinguished. 



There are some interesting experiments in this memoir, 

 on the colours of the shadows produced by the separation 

 of two lights procured from different sources, when com- 

 pared with each other ; from which it appears, that the 

 light reflected by the atmosphere is blue, when compared 

 with all artificial lights ; and that the light from hydrogen 

 or other combustibles in which it abounds, is blue, com- 

 pared with those in which there is a larger proportion of 

 carbon. In all these experiments, the shadows are com- 



