456 PHILOSOPHICAL TRANSACTIONS. [anNO 17Q5. 



that can attend so severe an operation ; these circumstances it is proper to men- 

 tion, as they contributed to render the patient a more favourable subject for ex- 

 periment. The man's name was Benjamin Clerk; he was a seafaring man, 21 

 years of age, and in perfect health. Both his eyes were free from complaint till 

 about April 1 1, 1793, at which time he was on a voyage home from the East 

 Indies, a sudden mist or dimness appeared before his right eye ; this increased 

 very rapidly, and on the 18th of the same month the sight was entirely obscured. 

 The crystalline humour was extracted on the 25th of Nov. ; and 27 days after the 

 operation the eye was so far recovered as to admit of the following observations 

 and experiments being made on it. 



In this man we had all the circumstances conibined, which seemed to be re- 

 quired to determine how far the crystalline lens was the principal agent in adjust- 

 ing the eye. The man himself was in health, young, intelligent, and his left 

 eye perfect; the other had been an uncommonly short time in a diseased state, and 

 appeared to be free from every other defect but the loss of the crystalline lens. He 

 very willingly allowed me to make the following experiments on him ; and re- 

 mained in town, though inconvenient to himself, till they were completed ; the 

 greater part of them were instituted by Mr. Ramsden, and all of them carried 

 through under his direction. The experiments were begun Dec. 22, 1793, at 

 which time the following observations were made on the imperfect eye. The eye 

 bore the light of the day very well ; but was fatigued by strong sunshine, or the 

 glare of candle-light. In weak lights objects were not seen at all by the imper- 

 fect eye, but in strong lights they presented a faint image, which appeared at the 

 same distance with that seen by the perfect eye, and close to it, or nearly so, but 

 always to the left. The imperfect eye, unassisted by glasses, could see objects, 

 but it was with a degree of indistinctness ; and this indistinct vision only took 

 place at a disttmce between 6 and 9 inches. With a double convex glass, the 

 radius of one surface 1 and -l- inch, of the other 6 inches, the flat side towards 

 the eye, having a focus of 24- inches, objects appeared most distinct at 4-^ inches, 

 and the extremes were 2^ inches, and 5^ inches. The different distances were 

 ascertained by placing one end of a foot rule against the man's forehead, and 

 giving him the book m his own hand, desiring him to carry it to the distance at 

 which he saw best, and afterwards to the 2 extremes of distinct vision, the upper 

 end of the book being always in contact with the rule ; so that the moment he 

 adjusted the book, the distance was read off" from the scale. The accuracy with 

 which he brought it to the same point in repeating the experiments, proved his 

 eye to be uncommonly correct ; fur as he did not liimself sec tlie scale, there could 

 be no source of fallacy. 



Making these experiments fatigued the eye considerably, and repeating them 



