THE SENSE OF SIGHT. 185 



These qualities are the diiferent sensations of color. "We distinguish white, rod, 

 blue, green, yellow light; we speak of white, red, and other colors of ihe ob- 

 jects which we sec — that is to say, we confer the special property of the sensa- 

 tion which we call color on the external object by which it is generated — with 

 the same incorrectness with which we attribute the properties of light in general 

 to external objects. Now, what this blue or green, this red or yellow color is, 

 can no one say ; we can as little describe the qualities of a sensation of light, as 

 the qualities of any impression received through the other senses; and how im- 

 possible this is has been sufficiently shown in previous essays. Jleanwhile the 

 true image of every sensation of color is indelibly fixed in all memories, so that 

 at any moment we can recall the idea of each several quality, or as>ign its ap- 

 propriate color to every object which presents itself; bnt no one can specify 

 any distinctive sign pertaining to the sensation itself, whether of blue or green ; 

 no one knows whether his own perception of blue accords with that of another 

 individual. In common life it is customary to speak of a black color, and even 

 in science it is sometimes doubted whether black should not be called a color ; 

 that is to say, a sensation. In my opinion there is no room for such a doubt. 

 Black is nothing but the absence of a perception of light. We call those ob- 

 jects black from which no light proceeds to our eye; we perceive them as gaps 

 in the illuminated field of vision, being conscious, by virtue of the sense of space 

 resident in the retina, of the particles of that membrane which remain in repose 

 and unexcited, between those particles which are struck and irritated by the 

 light. If we exclude all external light from our eyes, the whole field of vision 

 appears black ; we are conscious of the repose of all the particles of the retina. 

 But to enter more particularly into the details of this controversy, whether 

 black be a sensation or not, would lead us into too longr a digression. 



If we turn our consideration to the external causes of the different sensations 

 of color, so indescribable and inexplicable in themselves, we are met by an en- 

 tirely conclusive and exact physical theory. The different sensations of color 

 stand in relation to their causes in the ;-arae category with tiie scnsat ons of 

 sound of a different pitch ; it is iho, frequency of the vibrations of the pnrtic'es of 

 the luminous ether, the lengtk of its weaves, by which the quality of the sensa- 

 tion is determined ; to each definite color belongs a definite, exactly measured 

 length of wave and frequency of vibration, as to every sound of definite pitch be- 

 longs a definite number of vibrations of the body Avhich generates it. Physics 

 " has demonstrated that the white light of the sun — that is, the vibrations f)f the 

 ether proceeding from the sun which taking effect on the retina produce the 

 sensation of a white color — is no uncompounded light, but consists of a great 

 number of the ethereal vibrations differing in the degrees of their frequency ; it 

 has farther taught us to decompose this compound light into its several constit- 

 uent parts. If a sunbeam be leceived through the edge of a three-sided prism 

 of glass, while this edge, for example, is held before a small round opening in 

 an opaque screen upon which the sun shines, there will appear on a surface 

 situated behind the screen not a round white spot corresponding to the opening, 

 but a long bright stripe in which the different colors of the rainbow follow one 

 another in the same order as in the rainbow. One end of this str'pe will be 

 occupied by a red portion, followed by an orange, a yellow, a green, a blue, an 

 indigo and a violet colored portion in succession, the latter occupying the other 

 end of the stripe. Tiie origin of this prismatic stripe consists in this : that 

 the waves of different vibratory velocities c; impounded together in the white 

 light of the sun, on their way through the edge of the prism, are diverted, each 

 in a different degree, from their course, so that those waves in which the ethe- 

 real particles vibrate :uost swiftly are most diverted or refracted ; those whose 

 particles vibrate most slowly are least refracted ; if we imagine, for example, 

 the white solar ray striking horizontally on the prism, all the waves combined 

 therein will, when the edge of the prism is directed upwards, be so refracted as 



