6o Accou7it of some Experiments 



colours of the light by which they were produced, I 

 began to suspect that the colours of the shadows 

 might in many cases, notwithstanding their apparent 

 brilliancy, be merely an optical deception, owing to 

 contrast or to some effect of the other real and nei^h- 

 bouring colours upon the eye. 



To determine this fact by a direct experiment, I 

 proceeded in the following manner. Having, by mak- 

 ing use of a flat ruler instead of the cylinder, contrived 

 to render the shadows much broader, I shut out of the 

 room every ray of daylight, and prepared to make the 

 experiment with two Argand's lamps, well trimmed, 

 and which were both made to burn with the greatest 

 possible brilliancy ; and having assured myself that the 

 light they emitted was precisely of the same colour, by 

 the shadows being pefectly colourless which were pro- 

 jected upon the white paper, I directed a tube of about 

 12 inches long and near an inch in diameter, lined 

 with black paper, against the centre of one of the 

 broad shadows ; and looking through this tube with 

 one eye, while the other was closed, I kept my atten- 

 tion fixed upon the shadow, while an assistant repeat- 

 edly interposed a sheet of yellow glass before the lamp 

 whose light corresponded to the shadow I observed, 

 and as often removed it. 



The result of the experiment was very striking, and 

 fully confirmed my suspicions with respect to the fal- 

 lacy of many of the appearances in the foregoing ex- 

 periments. 



So far from being able to observe any change in the 

 shadow upon which my eye was fixed, I was not able 

 even to tell when the yellow glass was before the lamp 

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



