Experiments upon coloured Shadows. 115 
all the varieties of browns, and almost all the other colours I 
ever remembered to have seen, appeared in their turns, and 
there were even colours which seemed to me to be perfectly 
new. 
Reflecting upon the great variety of colours observed in 
these last experiments, many of which did not appear to have 
the least relation to the apparent 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 neighbouring colours upon the 
eye. To determine this fact by a direct experiment, I pro- 
ceeded in the following manner. Having, by making 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 Ar- 
gand'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 perfectly colourless which were 
projected upon the white paper, I directed a tube 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 attention fixed upon the shadow, while 
an assistant repeatedly interposed a sheet of yellow glass be- 
fore the lamp whose light corresponded to the shadow I 
observed, and as often removed it. The result of the experi- 
ment was very striking, and fully confirmed my suspicions 
