320 PHILOSOl'HICAL TRANSACTIONS. [aNNO 1793. 



would not act as he imagines, nor could they so act unobserved. The contrac- 

 tion of tlie ciliary zone is equally inadequate and unnecessary. 



Some have supposed the pressure of the external muscles, especially the 2 ob- 

 lique muscles, to elongate the axis of the eye. But their action would not be 

 sufficiently regular, nor sufficiently strong; for a much greater pressure being 

 made on the eye than they can be supposed capable of effecting, no sensible dif- 

 ference is produced in the distinctness of vision. Others say that the muscles 

 shorten the axis; these have still less reason on their side. Those who maintain 

 that the ciliary processes flatten the crystalline, are ignorant of their structure, 

 and of the effi^ct required: these processes are yet more incapable of drawing 

 back the crystalline, and such an action is equally inconsistent with observation. 

 Probably other suppositions may have been formed, liable to as strong objections 

 as those opinions here enumerated. 



From these considerations, and from the observation of Dr. Porterfield, that 

 those who have been couched have no longer the power of accommodating the 

 eye to different distances, I had concluded that the rays of light, emitted by ob- 

 jects at a small distance, could only be brought to foci on the retina by a nearer 

 approach of the crystalline to a spherical form, and I could imagine no other 

 power capable of producing this change than a muscularity of a part, or the 

 whole, of its capsule. But in closely examining, with the naked eye in a strong 

 light, the crystalline from an ox, turned out of its capsule, I discovered a structure 

 which appears to remove all the difficulties with which this branch of optics has 

 long been obscured. On viewing it with a magnifier, this structure became 

 more evident. 



The crystalline lens of the ox is an orbicular, convex, transparent body, com- 

 posed of a considerable number of similar coats, of which the exterior closely 

 adhere to the interior. Each of these coats consist of 6 muscles, intermixed 

 with a gelatinous substance, and attached to 6 membranous tendons. Three of 

 the tendons are anterior, 3 posterior; their length is about ^- of the semi-dia- 

 meter of the coat; their arrangement is that of 3 equal and equidistant rays, 

 meeting in the axis of the crystalline; one of the anterior is directed towards the 

 outer angle of the eye, and one of the posterior towards the inner angle, so that 

 the posterior are placed opposite to the middle of the interstices of the anterior; 

 and planes passing through each of the 6, and through the axis, would mark on 

 either surface 6 regular equidistant rays. The muscular fibres arise from both 

 sides of each tendon ; they diverge till they reach the greatest circumference of 

 the coat, and, having passed it, they again converge, till they are attached res- 

 pectively to the sides of the nearest tendons of the opposite surface. The ante- 

 rior or posterior portion of the 6 viewed together, exhibits the appearance of 3 



