VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 457 



after very short intervals made the eye water, and gave a slight degree of pain ; 

 this however soon went off. In looking at objects through this glass, the image 

 was free from any tinge of colour, unless he directed his eye towards the cir- 

 cumference of the glass, and then it had a considerable tinge, which evidently 

 arose from the prismatic figure of that part of the glass. A comparative expe- 

 riment was made on the perfect eye, with a glass of 15 inches focus. Objects 

 were found in one experiment to appear most distinct at 84- inches, the extremes 

 3 inches and 1 1 inches ; in another, most distinct at 7 inches, the extremes as 

 before, 3 and 1 1 inches. 



On Dec. 29, 34 days after the operation, the following experiments were made 

 by candle-light, about 6 o'clock in the evening. The experiment with the dou- 

 ble convex glass was repeated, the aperture being diminished to -^ of an inch ; 

 objects appeared most distinct at 5 inches, the extremes 3 inches and 7-2- inches. 

 The aperture was diminised to ^V of an inch, and vision appeared most distinct 

 at 5 inches, the extremes 3-1- inches and 7 inches. When the aperture was re- 

 duced to -jV of an inch, the inflexion of the rays produced the appearance of a 

 speck, which obscured his vision. By diminishing the aperture, s^nerical aber- 

 ration was in a great measure corrected, and vision rendered more distinct. 



A plano-convex glass of 2-|- inches focus, with the plane towards the eye, was 

 now applied, and the objects were most distinct at 6 inches, but by no means 

 well defined : the aperture was now reduced to .3-V of an inch, and objects ap- 

 peared much more distinct at 5-^ inches ; when the glass was brought within -i- 

 an inch of the eye, objects were still more distinct, and were seen at 5 inches. 

 The eye was less affected by these than the former experiments, nor was it fa- 

 tigued by the light of a candle. In strong lights a faint image was seen by the 

 imperfect eye, and always to the left of the other. The perfect eye, with a glass 

 of 15 inches focus, saw objects most distinctly at 8-^ inches, the extremes 3-i. 

 inches and lix inches. As these experiments were made with a view to deter- 

 mine whether the eye, when deprived of its crystalline humour, had a power of 

 adjusting itself to different distances ; that being ascertained, they were not pro- 

 secuted further, on account of the tender state of the man's eye, who went into 

 the country as soon as they were completed. 



On Nov. 4, 1 794, the man returned to London, and submitted himself to be the 

 subject of further experiments. This afforded us an opportunity of ascertaining 

 the comparative adjustment of the 2 eyes, when by means of different glasses 

 they were brought to see distinctly at nearly the same focal distance : an experi- 

 ment we had been unable to make before for want of proper glasses. Sir Henry 

 Englefield, who will be found to have given us his assistance in the subsequent 

 part of his investigation, was present at this experiment, and was much asto- 

 nished, as we had been in the former ones, at the accuracy with which the 



VOL. XVII. 3 N 



