460 ° PHILOSOPHICAL TRANSACTIONS. [aNNO 1795. 



drawn out and allowed to contract again, the change in the centre will be found 

 the most distinct ; the principal elasticity appearing to reside in that part. 



Before these experiments were made on the cornea, Mr. Ramsden had pro- 

 mised that he would contrive an instrument by which the cornea might be exa- 

 mined, while the eye was adapting itself to different distances; so as to enable us 

 to decide whether any change took, place at these times in its external figure. 

 When I state to the r. s. that 7 months elapsed before the apparatus for this ex- 

 periment was completed, they will not attribute it to a want of solicitude on my 

 part, or a want of attention in Mr. Ramsden ; but to delays which must neces- 

 sarily occur to an artist so extensively employed in business, and at the same 

 time so ready to engage both from inclination, and the urgent requests of his 

 friends, in promoting philosophical inquiries. 



On July 31, 1794, we were enabled to begin our experiments, for which the 

 following apparatus was constructed. A thick board was fixed to a strong up- 

 right support, directly opposite to the window of Mr. Ramsden's front room on 

 the first floor, which looks up Sackville-street, at the distance of 1 foot from 

 the window. In this board was a square hole, large enough to admit a person's 

 face, the forehead and chin resting against the upper and lower bars, and the 

 cheek against either of the sides, so that when the face was protruded, the head 

 was steadily fixed by resting on 3 sides, and in this position the left eye pro- 

 jected beyond the outer surface of the board. On the outside of the board, or 

 that next the window, on the left square hole, was fixed a microscope, so placed 

 as to take into its field the lateral part of the front of the cornea, which projects 

 beyond the eyelids. The microscope had not only a movement directly forwards 

 but by means of endless screws, had also a vertical and horizontal motion, with- 

 out which the experiments could not have been made with any degree of preci- 

 sion. From the upper part of the square hole a horizontal brass beam projected 

 towards the window, with joints, by which it could be lengthened or shortened ; 

 and at the end of this a brass plate was suspended, which admitted of being raised 

 or depressed, so as to bring a small hole that had been drilled through it directly 

 opposite to the eye. 



With this apparatus we began our experiments ; and I consider it as a fortu- 

 nate circumstance that Sir Henry Englefield arrived in town the night before 

 they were made; he very cheerfully gave his assistance the moment I made the 

 request. Sir Henry, from his practical knowledge of mathematical instruments, 

 and the habit of making observations with them, rendered us very material assist- 

 ance in the course of our experiments, and I feel myself obliged to him for re- 

 maining in town till they were completed. To Mr. Ramsden and myself it was 

 a particular satisfaction to have an evidence who had no presupposed opinion 

 therefore impartial ; whose knowledge of the subject enabled liim to form a 



