SEC. 6. ON COLOUR SENSATIONS. 



§ 558. The sensation excited by a luminous point possesses 

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

 stancy, and localization, 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 undula- 

 tions 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 undula- 

 tions having wave-lengths longer than that of the extreme red, 

 about \ 760, 1 or shorter than that of the extreme violet, about 

 \ 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 undula- 

 tions 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 con- 

 fined to the sensations other than those of white and black, some- 

 times it is used to comprise these two sensations as well. 



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



891 



