INTRODUCTION 195 



psychology. A more material analogy may be drawn from metabolism. 

 Here we have more or less accurate information as to the nature and 

 constitution of the food material supplied to an organ or to the body 

 and as to the nature and constitution of the secreted or excreted material. 

 The intermediate steps are full of gaps. In the domain of the special 

 senses physiology is the realm of the intermediate steps, physics and 

 psychology the termini. It is natural that the physicist should adopt 

 the method characteristic of his branch of science, the psychologist 

 that characteristic of his own. But if we survey the advance of psy- 

 chology in recent years we cannot but be struck by the fact that it 

 has been largely due to the adoption of the synthetic method. Experi- 

 mental psychology and the elaboration of " psycho-physical laws " are 

 results of this tendency^. 



It appears to me, therefore, that more credence is to be attached 

 to the results of the synthetic method. They alone are submissible 

 to approximately accurate quantitative estimates, and it is only by 

 quantitative estimates that the facts can be conclusively correlated. 

 We thus obtain a group of correlations which themselves attain the 

 dignity of precise knowledge. Outside this aristocracy of facts is a vast 

 multitude of undigested facts. Either they have not yet been sub- 

 mitted to accurate quantitative relationship or are as yet incapable of 

 so being. In so far as they fall into line with precise correlations they 

 are of confirmatory value. In so far as they are antagonistic they are 

 impotent to destroy these correlations, but ofier a well-stocked field 

 for further research. 



Not that they themselves are incapable of correlation, but it is 

 qualitative in nature and therefore less precise. Always remembering 

 their relative smaller value as coins of the realm these qualitative 

 correlations are well worthy of extended study. 



We have therefore in quantitative relations a criterion which can be 

 applied to the grouping of the facts which we have already brought 

 forward. If we rapidly survey the sections of Part I we shall find that 

 the quantitative relationships are best established for luminosity and 

 colour with the photopic fovea and for luminosity with the achromatic 

 scotopic eye. When we consider peripheral vision, temporal, and areal 

 effects, both for photopic and scotopic vision, the quantitative relations 

 are far less well established. The same grouping applies to Part II. 



Bearing in mind the relative precision of our knowledge of the 

 various facts we can at once hypothecate certain relationships with 



^ Cf. Myers, Experimental Psychology, Chap, i, London, 1909. 



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