THE EYE AS AN OPTICAL INSTRUMENT. 177 



observation and experiment. There is no longer much contro- 

 versy on the more important facts of observation, 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 num- 

 ber 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 sub- 

 ject how much labour must be expended to establish many facts 

 which appear 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 processes 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 apprehensions 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 that remarkable progress which ophthalmic 

 medicine has made during the last twenty years a progress 

 which for its rapidity and scientific character is perhaps without 

 parallel in the history of the healing art. 



Every lover of his kind must rejoice in these achievements 

 which ward off or remove so much misery that formerly we 

 were powerless to help, but a man of science has peculiar reason 

 to look on them with pride. For this wonderful advance has 

 not been achieved by groping and hicky finding, but by deduction 

 rigidly followed out, and thus carries with it the pledge of still 

 future successes. As once astronomy was the pattern from 

 which the other sciences learned how the right method will lead 



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