412 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



the activity of three independent physiological substances of un- 

 known nature and situation. (2) That the relationship between 

 these components and the complex of stimuli is expressible 

 quantitatively by saying that the responsiveness of each component 

 is measured by a real linear function of three standard stimuli. 



(3) The results of stimulating these components are unit sensa- 

 tions in a purely physiological sense, not units of consciousness. 



(4) No spectral light acts upon only one hypothetical component. 

 The theory describes with sufficient accuracy the main facts, 



and there is some direct experimental evidence that of Burch 

 which is consistent with its truth. Further, the main objection 

 to the hypothesis in its modern form is its highly general nature 

 and want of direct applicability to the immediate objects of 

 physiological and physical research. How far this is a real 

 objection may be a matter of discussion, but it at least in- 

 clines one to examine those theories which are, in the colloquial 

 phrase, less up in the clouds. Such an examination will now 

 occupy us. 



SECTION VI. BERING'S THEORY OF VISUAL SENSATIONS 



In the preceding section, I attempted to trace out the theo- 

 retical consequences which it seemed possible to reconcile with 

 the experimental facts of colour mixing. As I have pointed out, 

 with wearisome frequency, perhaps, the whole process depended 

 upon the observation that, in general, the effect of a given stimulus 

 or of a combination of stimuli was constant, so that we might 

 attribute to the stimulus a causal value. In other words, we have 

 regarded the sensations as the signs or differentia of stimulation 

 processes, and our theory, therefore, was in that sense not a theory 

 of visual sensations at all but a theory of visual stimuli. 



If the estimate I formed of the value of this process be at all 

 just, it would follow that its weakness lay rather in what was 

 left unsaid than in any incorrectness or improbability in its positive 

 teaching. An objector, let us say the hypothetical man in the 

 street, might perhaps express his criticism in the following terms : 

 " You compel me," he might say, " to examine a great many facts, 

 and force me to puzzle out some difficult quantitative reasoning, and 

 at the end you leave me with a few highly general theorems which, 

 you confess, only describe some of the phenomena. Whenever I 



