108 KECEXT PEOGRESS OF THE THEORY OF VISION. 



the results of which are involved in those acts of appre- 

 hension 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 ac- 

 customed to regard the apprehension 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 en- 

 trance 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 natm-e of the mental 

 processes which contribute to the result. I will therefore 

 endeavour to give some account of the results of physi- 

 ological inquiries so far as they bear on the questions 

 above mentioned. 



I am the more desirous of doing so because I have 

 lately completed ' a complete survey of the field of physio- 

 logical optics, and am happy to have an opportunity of 



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

 Leipzig in 1867. 



