VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. . 463 



most trifling things may interfere with the result of the experiment, and that it 

 required a considerable degree of nicety and management in adjusting the instru- 

 ment, without which the experiment could not have been made. 



August the '28th, the former experiments were repeated by Sir Henry Engle- 

 field, Mr. Ramsden, and myself, on the eye of a young lad, and the result was 

 similar to the others, the motion of the cornea was uncommonly distinct. Sir 

 Henry now became the subject of the experiment, and changed the adjustment 

 of his eye from one distance to another in a very irregular manner, without 

 giving the smallest information, with a view to embarrass Mr. Ramsden who was 

 the observer, but without effect, for Mr. Ramsden was able to tell every change 

 in distance he had made, without a single mistake ; this exceeded our expectation, 

 and appeared to us so satisfactory that we required no further proofs of the 

 truth of our former observations. Before we concluded our experiments, every 

 mode that could be devised was put in practice to see how far there might be any 

 deception ; the eye was moved on its axis, and in different directions, but these 

 motions did not give at all similar appearances to those seen in the adjusting of 

 the eye to different distances. 



From the different experiments which I have had the honour to lay before 

 the R. s., I shall consider the following facts to have been ascertained. 1st, 

 That the eye has a power of adjusting itself to different distances when deprived 

 of the crystalline lens ; and therefore the fibrous and laminated structure of that 

 lens is not intended to alter its form, but to prevent reflections in the passage of 

 the rays through the surfaces of media of different densities, and to correct 

 spherical aberration. 2d, That the cornea is made up of laminae ; that it is elastic, 

 and when stretched, is capable of being elongated -^ part of its diameter, con- 

 tracting to its former length immediately on being left to itself 3d, That the 

 tendons of the 4 straight muscles of the eye are continued on to the edge of 

 the cornea, and terminate, or are inserted, in its external lamina ; their action 

 will therefore extend to the edge of the cornea. 4th, That in changing the 

 focus of the eye from seeing with parallel rays to a near distance, there is a vi- 

 sible alteration produced in the figure of the cornea, rendering it more convex ; 

 and when the eye is again adapted to parallel rays, the alteration by which the 

 cornea is brought back to its former state is equally visible. 



Having supported these facts by the evidence of anatomical structure, and ab- 

 solute demonstration, I shall consider them to be established ; and make some 

 observations on the muscular and elastic power by which so very curious an effect 

 as the adjustment of the eye is produced. The 4 straight muscles of the eye 

 are attached to the bottom of the bony orbit near the foramen opticum ; they 

 become broader as they pass forward, and when arrived at the anterior part of 

 the eye-ball, are insensibly changed for tendons ; these adhere to the sclerotic 



