1313.] 



i?i the Eyes of Birds, 



m 



suggest the things which they signify, not by any likeness or 

 identity of nature, but only by an habitual connection, which 

 constant experience has proved to subsist between them ; nor is 

 the supposition, which attributes to the eye the faculty of seeing 

 objects distinctly by rays which do not accurately converge upon 

 the retina, altogether without foundation, since M. de la Hire* 

 has demonstrated that " the eye does not change its conforma- 

 tion, or adapt itself to the distances of objects, when they are 

 viewed through a perforated card." For if a small object, 

 placed at that distance from the eye at which vision is most 

 distinct, be viewed through three pin-holes, so disposed that the 

 interval between the most distant of them shall not exceed the 

 diameter of the pupil, the object will be seen single; but if the 

 object be brought either within or beyond the limits of distinct 

 vision, it will be seen multiplied as many times as there are 

 holes in the card, and each of the three images will be as 

 perfect as the single one. It is obvious that the three images are 

 formed by three pencils of rays, which are cut by the retina, 

 either before their convergence or after their decussation, and 

 consequently the object is seen distinctly by rays which do not 

 accurately converge upon the retina. Now, altSiough there is 

 every reason to distrust M. de la Hire's general conclusion, that 

 the eye is not adapted to the different distances of objects by any 

 change in its optical conformation, still I believe it will be 

 readily admitted that if the reality of such a change be at all 

 questionable, the hypotheses, which have been contrived to 

 explain the means by which it is effected, cannot be received as 

 proofs of its existence. Sir Everard Home and Mr. Ramsden, 

 in the true spirit of philosophical research, endeavoured to bring 

 the rbatter to the test of experiment; but the only conclusion, 

 which those philosophers felt themselves authorised to draw from 

 experiments conducted with a degree of accuracy of which the 

 subject seemed scarcely susceptible, was, " that a change in 

 the length of the axis of vision, for the purpose of adjusting the 

 eye, is rendered highly probable ;"t but since,' if it happened at 

 all, it could not exceed -§--5-0^^^ P^rt of an inch, and as such a 

 change would not account for the phenomena, it became neces- 

 sary to introduce the agency of other causes, which, as they 

 could not be subjected to the test of experiment, must be consi- 

 dered as merely hypothetical. The strongest argument in favour 

 of the internal changes of the eye seems to be drawn from 

 comparative anatomy ; for if it be true that there is in the eyes 

 of birds an organ which regulates the focal distance of the 



* M. de la Hire, Journal des Sjavans, 1693. Porterfield on the internal 



icotions of the eye. 



t Crooniau Lecture, Phil, Trans. 1796. 



