PREFACE 



rpHE vast literature on colour vision consists almost entirely of 

 -^ papers written in support of some particular theory. It is 

 peculiarly difficult to obtain a general and unbiassed view of the 

 subject. I have here endeavoured to separate the best established 

 facts of colour vision from the theories, and have then discussed the 

 chief theories in the light of these facts. 



I wish to express my great indebtedness to Sir WilUam Abney, 

 K.C.B., F.R.S., Professor W. Watson, F.R.S., and Dr Myers for their 

 invaluable assistance in a very difficult task, which demands a not 

 inconsiderable knowledge of such diverse subjects as physics, physiology, 

 and psychology. The chief references to the literature have been given 

 in footnotes. The frequent references to " Abney " are to his Researches 

 in Colour Vision (London, 1913), and to " v. Helmholtz" to his Hand- 

 buch der physiologischen Optik. In the latter case each of the three 

 editions contains material which is absent from the others, and the 

 exact reference is given in the footnote. 



Note. Fig. 12, p. 47. The discrepancy in these curves has been 

 found to be due to a technical defect in the apparatus. More recent 

 observations show the identity of the curves obtained by the two 

 methods, thus confirming Ives's results (Fig. 38). 



J. H. P. 



November, 1914 



/ G zt 



