and Coloured Shadows. 437 



coloured shadows, whicli are complementary to each other, 

 whether formed hy light different in colour, in the different 

 directions in which it arrives, or rendered so by the coloured 

 objects that reflect it within the apartment in which the 

 shadows are seen. 



The third class is not often produced except by artificial 

 means. The effect in the experiments belonging to this 

 class, to which we have before referred, may be consider- 

 ably heightened, by looking at the same time, with the 

 dift'erent eyes, through two glasses complementary to each 

 other, green and crimson, for instance ; the distinguishing- 

 colours will quickly be followed by an uniform gray, more 

 or less dark, as the glasses exclude more or less of the 

 coloured rays; but upon removing the glasses, and looking 

 at a white object alternately with the eyes thus differently 

 impressed, the complementary colours will be very distinct: 

 or a still greater effect may be produced, by quickly chang- 

 ing the glasses after the eyes have been sufficiently im- 

 pressed, when the gray will be converted to its vivid com- 

 plementary colours ; which, however, will soon be lost in 

 the uniform appearance they previously presented. Nothing 

 can more clearly prove the gradually lessened sensibility of 

 the eyes, when exposed to any prevailing colour, than these 

 experiments. 



I shall now make a few remarks upon some other 

 subjects adverted to in Mr. Tomlinson's paper ; but having 

 already very far exceeded the limits I had prescribed to 

 myself, I shall confine them to such points as are connected 

 with the explanation of my own views. 



Mr. Tomlinson, in his first experiment, states, that if 

 the green disk be instantly re-placed, after looking at its 

 accidental colour upon a sheet of white paper, the eye will 

 be unable to distinguish any thing for a second or two 

 previously to the re-appearance of the green ; and he 

 accounts for this, by supposing that the super-position of 

 the three primitive colours produce black. 



I have never been able to observe this interval of dark- 

 ness, although I have been led to expect it upon theoretical 

 principles. When the eye is impressed with green, so as 

 to be rendered insensible to this colour, and then turns to 

 a white surface, its sees only the colours reflected by it, 



