176 RECENT PROGRESS OF THE THEORY OF VISION. 



the results of which are involved in those acts of apprehension 

 by the senses which at first sight appear to be most simple 

 and immediate. These concealed functions have been but little 

 discussed, because we are so accustomed to regard the appre 

 hension of any external object as a complete and direct whole, 

 which does not admit of analysis. 



It is scarcely necessary for me to remind my present readers 

 of the fundamental importance of this field of inquiry to almost 

 every other department of science. For apprehension by the 

 senses supplies after all, directly or indirectly, the material of all 

 human knowledge, or at least the stimulus necessary to develope 

 every inborn faculty of the mind. It supplies the basis for the 

 whole action of man upon the outer world ; and if this stage of 

 mental processes is admitted to be the simplest and lowest of its 

 kind, it is none the less important and interesting. For there 

 is little hope that he who does not begin at the beginning of 

 knowledge will ever arrive at its end. 



It is by this path that the art of experiment, which has 

 become so important in natural science, found entrance into the 

 hitherto inaccessible field of mental processes. At first this will 

 be only so far as we are able by experiment to determine the 

 particular sensible impressions which call up one or another 

 conception in our consciousness. But from this first step will 

 follow numerous deductions as to the nature of the mental pro- 

 cesses which contribute to the result. I will therefore endeavour 

 to give some account of the results of physiological inquiries so 

 far as they bear on the questions above mentioned. 



I am the more desirous of doing so because I have lately 

 completed 1 a complete survey of the field of physiological optics, 

 and am happy to have an opportunity of putting together in a 

 compendious form the views and deductions on the present sub- 

 ject which might escape notice among the numerous details of a 

 book devoted to the special objects of natm-al science. I may 

 state that in that work I took great pains to convince myself of 

 the truth of every fact of the slightest importance by personal 



1 Prof. Helmholtz's Handbook of Physiological Optics was published at 

 Leipzig in 1867. 



