THE EYE AS AN OPTICAL INSTRUMENT. 199 



putting together in a compendious form the views and 

 deductions on the present subject which might escape 

 notice among the numerous details of a book devoted to 



O 



the special objects of natural 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 observation and experiment. There is no longer 

 much controversy on the more important facts of obser- 

 vation, the chief difference of opinion being as to the 

 extent of certain individual differences of apprehension 

 by the senses. During the last few years a great number 

 of distinguished investigators have, under the influence 

 of the rapid progress of ophthalmic medicine, worked at 

 the physiology of vision ; and in proportion as the 

 number of observed facts has increased, they have 

 also become more capable of scientific arrangement and 

 explanation. I need not remind those of my readers 

 who are conversant with the subject how much labour 

 must be expended to establish many facts which appeal- 

 comparatively simple and almost self-evident. 



To render what follows understood in all its bearings, 

 I shall first describe the physical characters of the eye 

 as an optical instrument ; next the physiological pro- 

 cesses of excitation and conduction in the parts of the 

 nervous system which belong to it ; and lastly I shall 

 take up the psychological question, how mental appre- 

 hensions are produced by the changes which take place in 

 the optic nerve. 



The first part of our inquiry, which cannot be passed 

 over because it is the foundation of what follows, will 

 be in great part a repetition of what is already generally 

 known, in order to bring in what is new in its proper 

 place. But it is just this part of the subject which 

 excites so much interest, as the real starting point of 



