66o 



THE SENSE OF SIGHT. 



was dark may appear light. This occurs whenever the 

 eye, which is the seat of the spectrum of a luminous object, 



Fig. 187.' 



is not closed, but fixed 

 upon another bright or 

 white surface, as a white 

 wall, or a sheet of white 

 paper. Hence the spectrum 

 of the sun, which, while 

 light is excluded from the 

 eye is luminous, appears 

 black or grey when the 

 eye is directed upon a 

 white surface. The explanation of this is, that the part 

 of the retina which has received the luminous image 

 remains for a certain period afterwards in an exhausted or 

 less sensitive state, while that which has received a dark 

 image is in an unexhausted, and therefore much more 

 excitable condition. 



The ocular spectra which remain after the impression of 

 coloured objects upon the retina are always coloured ; and 



* Fig. 187. A circle showing the various simple and compound 

 colours of light, and those which are complemental of each other, i.e., 

 which, when mixed, produce a neutral grey tint. The three simple 

 colours, red, yellow, and blue, are placed at the angles of an equilateral 

 triangle ; which are connected together by means of a circle ; the mixed 

 colours, green, orange, and violet, are placed intermediate between the 

 corresponding simple or homogeneous colours ; and the complemental 

 colours, of which the pigments, when mixed, w r ould constitute a grey 

 and of which the prismatic spectra would together produce a white light. 

 will be found to be placed in each case opposite to each other, but con- 

 nected by a line passing through the centre of the circle. The figure 

 is also useful in showing the further shades of colour which are com- 

 plementary of each other. If the circle be supposed to contain every 

 transition of colour between the six marked down, those which, when 

 united, yield a white or grey colour, will always be found directly 

 opposite to each other ; thus, for example, the intermediate tint between 

 orange and red is complemental of the middle tint between green and 

 blue. 



