PART I 



THE CHIEF FACTS OF NORMAL 

 COLOUR VISION 



SECTION I 



THE BASES OF COLOUR VISION 

 CHAPTER I 



THE PHYSICAL BASIS 



What is generally understood by the term " light " is a composite 

 congeries of allied manifestations of energy, comprising such apparently 

 various phenomena as heat, Hght in the narrower sense of the word, 

 and chemical action. Various as these phenomena are, they are 

 physically identical in character, all consisting of radiant energy in the 

 form of waves of identical character, differing only in the length and 

 rapidity of the vibrations. Broadly speaking, the longest waves cause 

 the sensation of heat, the shortest give rise to chemical action, whilst 

 those of intermediate length cause the sensation of light. 



If we take ordinary sunlight as the basis of our investigations, it is 

 possible to split it up by appropriate means into its component "rays," 

 differing from each other in wave-length. Of these certain are visible, 

 and constitute light in the narrower sense of the word, but instead of 

 giving rise to the sensation of white light, they, to the majority of 

 people, show certain pure colours, viz. red, orange, yellow, green, blue, 

 and violet, in order, the red having the longest and the violet the shortest 

 wave-length. The visible spectrum extends from about 723 ixfi at the 

 red end to 397ju,/li at the violet end. v. Helmholtz under the most 

 favourable conditions was able to see as far as about 8^5 [x/jl. The 

 limitation of the spectrum at the violet end is less precise, because the 

 rays in this neighbourhood are changed into rays of greater wave-length 

 p. c. V. 1 



