PART III 



THE CHIEF THEORIES OF COLOUR VISION 



SECTION I 



GENERAL REVIEW 

 CHAPTER I 



INTRODUCTION 



In the previous parts I have endeavoured to set forth the best- 

 established facts of normal and abnormal colour vision. There are 

 other equally well established facts, but they are so intimately associated 

 with the theories which we have now to consider that it has been thought 

 better to defer consideration of them. We have already, however, 

 accumulated sufficient to warrant an attempt at this stage to classify 

 them. Such as have been mentioned form the basis of the chief theories 

 of colour vision and will enable the reader to obtain a grasp of the 

 significance of these theories. 



No theory has ever been propounded which suffices to explain all 

 the facts of the department of science to which it refers. If it succeeds 

 in offering a complete solution of all the problems it ceases to be a theory 

 and merits at least as secure a position in the domain of knowledge 

 as the facts themselves. As McDougall^ says, " the most fruitful 

 hypothesis ever developed is perhaps that of an ethereal medium to 

 which an impossible and inconceivable combination of properties is 

 assigned by physicists." Theories must be judged solely according to 

 their efficacy as working hypotheses. In so far as they serve the 

 purpose of sign-posts, pointing out the paths of future research, so far 

 are they of value. " Sterile theories easily relinquish immortality. 

 Fruitful theories hand down their immortal part to their children, while 

 their epheiueral shell falls to pieces " (Hering)^. 



1 Brain, xxiv. 611, 1901. 2 L^iog^ n. f. i. 15, I88O. 



P. c. V. 13 



