256 ON LIGHT. 



veying to us information not only of the forms and situa- 

 tions of objects, but of all that multitude of their physical 

 properties which stand in relation to colour, both those 

 which ordinary experience teaches and which science 

 reveals. 



(39.) Lastly, by thus reuniting into one beam rays 

 going to form distant portions of the spectrum, and ex- 

 cluding the rest, we find that it is possible to produce a 

 compound^ beam which shall excite directly in the <. ye, 

 or illuminate a screen with any one of the innumerable 

 varieties of tint which we observe in nature ; and what 

 is especially remarkable, the same tint, or one undis- 

 tinguishable from it to ordinary eyes, is producible by 

 very different combinations of the prismatic rays ; while 

 yet there exist individuals, and these not unfrequent, 

 who are perfectly capable of discriminating (in many 

 cases) between such compound tints, and who even 

 declare them to be widely different. To such cases of 

 what is called, though improperly, "colour-blindness," 

 we shall presently have occasion to recur. 



(40.) The consideration of these facts has given rise 

 to a speculation which, if not demonstrable, has at least 

 a high degree of plausibility, and which, at all events, 

 has never yet been ^proved, viz.,, that there is no 

 real connexion between COLOUR and REFRANGIBILITY, 

 but that there exist three inherently distinct species of 

 light, each competent per se to excite the sensation of 

 one of three PRIMARY COLOURS, by whose mixture all 

 compound tints are produced, white consisting of their 

 totality, and black being the exponent of their entire 



