ON LIGHT. 



257 



absence. That, mojeover, each of them has a spectrum 

 of its own, over the whole length of which it is dis- 

 tributed according to its own pecuhar law of intensity^ 

 and from whose superposition on the same ground re- 

 sults the prismatic spectrum, coloured as we see it. 

 The annexed figure will convey a better conception of 



Fig. 3- 



this than any lengthened description, where A B repre- 

 sents the length of the total spectrum wherewith each 

 of the three is co-extensive, and where the curved lines 

 marked r, g, b, severally express, by the heiglit to 

 which they rise on any one point in a b, the intensity 

 in Its own spectrum of each of the primary colours ; 

 while the dotted curve, whose ordinate or height cor- 

 responding to any point is the sum of those of the 

 other curves, will of course express the joint intensity 

 or degree of illumination in the visible spectrum. 



(41.) In this view of the subject, the prismatic 

 colours, with the exception of the extreme red, are all 

 more or less mixed tints, and this agrees well with its 

 general aspect, in which the red and indigo-blue are 

 the only full and pure tints, the green being by no 

 means a saturated or full green, and the violet having 

 a strong dash of purphsh-red in it. 



