30'2 On the Mechanlfm of the Eye. 



I afterwards provided a fmall optometer, with a lens of lefs than two inches focus, 

 adding a feries of letters, not in alphabetical order, and proje£led into fuch a form as to 

 be moft legible at a fmall inclination. The excefs of the magnifying power had the advan- 

 tage of making the lines more divergent, and their croHing more confpicuous; and the 

 Jetters ferved for more readily naming the diftance of the interfeftion, and, at the fame 

 time, forjudging of the extent of the power of diflinguifliing objects too near or too re- 

 mote for perfeft vifion. (Plate XV. Fig. 23.) 



2. Mr. J. had not an eye very proper for the experiment ; but he appeared to diftlnguifli 

 the letters at 2-i inches, and at lefs than an inch. This at firft perfuaded me, that he muft 

 have a power of changing the focal diftance : but I afterwards recollected that he had 

 withdrawn his eye confiderably, to look at the nearer letters, and had alfo partly clofed his 

 eyelids, no doubt contradting at the fame time the aperture of the pupil ; an a(^ion which, 

 even in a perfect eye, always accompanies the change of focus. The flider was not 

 applied. 



3. Mifs H. a young lady of about twenty, had a very narrow pupil, and I had not an 

 opportunity of trying the fmall optometer : but, when fhe once faw an objedt double 

 ■through the flits, no exertion could make it appear fitrgle at the fame diftance. She ufed 

 for diftant objedls a glafs of 4I inches focus; with this fhe could read as far off as 12 

 inches, and as near as five : for nearer obje£ls flie added another of equal focus, and could 

 tlicn read at 7 inches, and at 2~. 



4. Hanfon, a carpenter, aged 63, had a catara£t extraSled a few years fince from one 

 eye : the pupil was clear and large, and he faw well to work with a lens of 2|- inches 

 focus; and could read at 8 and at 15 inches, but moft conveniently at 1 1-. With the fame 

 glafs, the lines of the optometer appeared always to meet at 1 1 inches -, but he could not 

 perceive that they crofled, the line being too ftrong, and the interfeftion too diftant. 

 The experiment was afterwards repeated with the fmall optometer : he read the letters from 

 2 to 3 inches; but the interfeftion was always at 2^ inches. He now fully underftood 

 the circumftances that were to be noticed, and faw the crofting with perfedl diftinftnefs : 

 zt one time, he faid it was a tenth of an inch nearer; but I obferved that he had removed 

 his eye two or three tenths from the glafs, a circumftance which accounted for this fmaU 

 difference. 



5. Notwithftandifig Hanfon's age, I confider him as a very fair fubjedt for the ex- 

 periment. But a ftill more unexceptionable eye was that of Mrs. Maberly. She is about 

 30, and had the cryftalline of both eyes extracted a few years fince, but fees beft with her 

 light. She walks without glafles ; and, with the affiftance of a lens of about four inches 

 focus, can read and work with eafe. She could diftinguifli the letters of the fmall opto- 

 meter from an inch to 2^ inches ; but the interfeftion was invariably at the fame point, 

 about 19 tenths of an inch diftant. A portion of the capfule is ftretched acrofs the pupil, 

 and caufcs her to fee remote obje£ts double, when without her glafles j nor can flie, by 



any 



