SEC. 8. ON COLOUR SENSATIONS. 



755. The sensation excited by a luminous point possesses 

 still another character besides those of intensity, duration, con- 

 stancy, and localisation, namely the one which we speak of as 

 colour. 



When we allow sunlight reflected from a white cloud or from 

 a sheet of white paper to fall into the eye, we have a sensation 

 which we call that of white light. When we look at the same 

 light through a prism and allow different parts of the spectrum to 

 fall in succession into the eye, we have a series of sensations, 

 differing in character from the sensation of white light and from 

 each other ; these we call ' colour sensations,' sensations of red, 

 yellow, and the like. In the latter case the luminous undulations 

 are dispersed in a linear series according to their wave-lengths, 

 from the short waves of the extreme violet to the long waves of the 

 extreme red ; and we learn from the spectrum, on the one hand, 

 that undulations having different wave-lengths produce different 

 sensations, and on the other hand that undulations having wave- 

 lengths longer than that of the extreme red, about X 760 \ or 

 shorter than that of the extreme violet, about X 390, are unable to 

 excite the retina and are therefore invisible. When we look 

 directly at a white object all this dispersion is absent, and the 

 retina is excited at the same time by undulations of all the above 

 wave-lengths. A sensation of ' colour ' then is a sensation evoked 

 by undulations of particular wave-lengths, a sensation of ' white ' 

 is the sensation which results when the retina, or a part of it, 

 is simultaneously excited by undulations of all wave-lengths 

 which are able to affect it, that is by the whole visible spectrum. 

 When we direct our eyes to an object in such a way that the rays 

 of light proceeding from it might fall on the retina when we 

 bring the object within our field of vision, and yet experience 

 from it neither any sensation of white nor any of the various 

 colour sensations, we call the resulting affection of consciousness 

 a sensation of ' black,' we say that we see ' black.' Sometimes 

 the word ' colour ' is confined to the sensations other than those 



1 X signifies a millionth of a millimeter or '001 p.. 



