66-4 PHILOSOPHICAL TRANSACTIONS. [aNNO 1790. 



the same size, and to keep the cornea and lens at the same distance from each 

 other. The ciliary processes, as they form a complete septum between the vi- 

 treous and aqueous humours, must be moved forward, together with the lens, 

 when the cornea is rendered more convex, and when the cornea recovers itself 

 they are thrown back into their former situation. In order to effect this with 

 the nicety that is required, the ciliary processes are probably possessed of a mus- 

 cular power. 



That the ciliary processes are muscular is a very generally received opinion, 

 and in the course of this lecture I shall adduce some facts in favour of it ; they 

 will also tend to confirm the opinion of these processes being a sling, in which 

 the lens is suspended, and rendered capable of a small degree of motion. The 

 result of this inquiry, which has not been confined to the support of any parti- 

 cular theory, but carried on with the sole view of discovering the truth, appears 

 to be, that the adjustment of the eye is produced by 3 different changes in that 

 organ ; an increase of curvature in the cornea, an elongation of the axis of vi- 

 sion, and a motion of the crystalline lens. These changes in a great measure 

 depend on the contraction of the 4 straight muscles of the eye. Mr. Ramsden 

 has made a computation, by which the degree of adjustment produced by each 

 of these changes may be ascertained. This he has promised to render more 

 correct ; and also to institute a series of experiments by which the effects of the 

 motion of the lens may be more accurately determined. From Mr. Ramsden's 

 computation, the increase of curvature of the cornea appears capable of pro- 

 ducing -i- of the effect ; and the change of place of the lens, and elongation of 

 the axis of vision, sufficiently account for the other ^ of the quantity of adjust- 

 ment necessary to make up the whole. 



Having explained the mode by which the axis of vision can be elongated, and 

 the convexity of the cornea increased, in the human eye, for the purpose of its 

 adjustment, I was desirous of applying these observations to the eyes of other 

 animals, that I might see whether their different structures would admit of the 

 necessary changes, for producing an adjustment to different distances in the 

 same way. As many animals are known to have their vision distinct at very 

 different distances, it appeared that much information might be gained by exa- 

 mining the structure of the eyes of those whose range of vision varies most 

 from that of the human eye. Quadrupeds in general must have their eyes fitted 

 to see very near objects, as many of them collect their food with their mouths, 

 in which action the objects are brought very close to the eye. Birds are under 

 the same circumstances in a still greater degree with respect to their food ; but 

 from their mode of life, they also require the power of seeing objects at a great 

 distance. Fishes, from the nature of the medium in which they live, must have 

 some other mode of adjusting the eye, than that of a change in the cornea, as 



