592 



ON THE MECHANISM OF THE ETE. 



Home and Mr, Ramsden. But either of the 

 series of experiments, which have been re- 

 lated, appears to be sufficient to confute it. 



IX. It now remains to inquire into the pre- 

 tensions of the crystalline Jens to the power of 

 altering the focal length of the eye. The 

 grand objection, to the efficacy of a change of 

 ligure in the lens, was derived from the ex- 

 periments, in which those, whohavebeen de- 

 prived of it, have appeared to possess the fa- 

 culty of accommodation. 



My friend Mr. Ware, convinced as he was 

 of ihe neatness and accuracy of the experi- 

 mems iclated. in the Croonian Lecture for 

 179.3, yet could not still help imagining, 

 iVoin the obvious advantage ail his patients 

 found, after the extraction of the lens, in 

 using two kinds of spectacles, that there must, 

 in such cases, be a deficiency in that faculty. 

 This circumstance, combined with a consi- 

 deration of the directions very judiciously 

 given by Dr. Porterfield, for ascertaining 

 the point in question, first made me wish to 

 repeat the experiments upon various indivi- 

 duals, and with the instrument which i have 

 above described, as an improvement of Dr. 

 Porterfield's optometer : and I must here ac- 

 knowledge my great obligation to Mr. Ware, 

 for the readiness and liberality, with which he 

 introduced me to such of his numerous pa- 

 tients, as he thought most likely to furnish a 

 satisfactory determination. It is unnecessary 

 to enumerate every particular experiment ; 

 but the universal result is, c-ontrarily to the 

 expectation with which lentered on the in- 

 quiry, tliat, in an eye deprived of the crystalline 

 lens, the actual focal distance is totally un- 

 changeable. This will appear from a selec- 

 tion of the most decisive observations. 



1, Mr. R. can read at four inches and at 



six only, with the same glass. He saw the 

 double lines meeting at three inches, and al- 

 ways at the same point ; but the cornea was 

 somewhat irregularly prominent, and his vi- 

 sion not very distinct; nor had I, at the time 

 that I saw him, a convenient apparatus. 



I afterwards provided a small optometer, 

 with a lens of less than two inches focus, add- 

 ing a series of letters, not in alphabetical 

 order, and projected into such a form as to, be 

 most legible at a small inclination. The ex- 

 cess of the magnifying power had the advan- 

 tage of making the lines more divergent, and 

 tlieircrossingmore conspicuous; and theletters 

 served fur more readily naming the distance of 

 the intersection, and, at the same time, for 

 judging of the extent of the power of distin- 

 guisiiing objects, too near, or too remote, for 

 perfect vision. (Plate 11. Fig. 87.) 



2. Mr. J. had not an eye very proper for 

 the experiment ; but he appeared to distin- 

 guish the letters at 24^ inches, and at less 

 than an inch. This at first persuaded me, 

 that he must have a power of changing the 

 focal distance: but I afterwards recollected 

 that he had withdrawn his eyeconsiderablv, to 

 look at the nearer letters, and had also partly 

 closed his eyelids, no doubt contracting at 

 the same time the aperture of. the pupil ; an 

 action which, even in a perfect eye, always 

 accompanies the change of focus. The 

 slider was not applied. 



3. Miss H. a young lady of about twenty, 

 had a veiy narrow pupil, and I had not an 

 opportunity of trying the small optometer; 

 but when she once saw an object double 

 through the slits, no exertion could make it 

 appear single at the same distance. She 

 used for distant objects a glass of 4i inches 

 focus; with this she could read as far off as 



