524 



OBSERVATIONS ON VISION. 



bute, in his Dioptrics, any material effect to 

 this change: in his Treatise on Man, how- 

 ever, he explains theoperation very minutely. 



De la Hire maintains, that the eye un- 

 dergoes no change, except the contraction 

 and dilatation of the pupil. He does not at- 

 tempt to confirm this opinion by mathema- 

 tical demonstration ; he solely rests it on an 

 experiment, which has been shown by Dr. 

 Porterfield and by Dr. Smith to be fallacious. 

 Hallertoo has adopted this opinion, however 

 inconsistent it seems with the known princi- 

 ples of optics, and with the slightest regard to 

 hourly experience. • 



Dr. Pemberton supposes the crystalline to 

 contain muscular fibres, by which one of its 

 surfaces is flattened while the other is made 

 more convex. But Dr. Jurin has proved that 

 a change like this is inadequate to the effect. 



Dr. Porterfield conceives, that the ciliary 

 processes draw forwards the crystalline, and 

 make the cornea more convex. The ciliary 

 processes are, from their structure, attach- 

 ment, and direction, utterly incapable of 

 this action; and, by Dr. Jurin's calculations, 

 there is not room for a sufficient motion of 

 this kind, without a very visible increase in 

 the length of the eye's axis : such an increase 

 we cannot observe. 



Dr. Jurin's hypothesis is, that the uvea, at 

 its attachment to the cornea, is muscuhu-, 

 and that the contraction of this ring makes 

 the cornea more convex. He says, that the 

 fibres of this muscle may as well escape our 

 observation, as those of the muscle of the in- 

 terior ring. But if such a muscle existed, 

 it must, to overcome the resistance of the 

 coats, be far stronger than that which is only 

 destined to the uvea itself ; and the uvea, at 

 this part, exhibits nothing but radiated fibres, 

 losing themselve?, before the circle of adhe- 



rence to the sclerotica, in a brownish granu- 

 lated substance, not unlike in appearance 

 to capsular ligament, common to the uvea 

 and ciliary processes, but which may be 

 traced separately from them both. Now at 

 the interior ring of the uvea, the appearance 

 is not absolutely inconsistent with the pre- 

 sence of an annular muscle. His theory of 

 accommodation to distant objects is in- 

 genious, but no such accommodation takes 

 place. , - , 



Musschenbroek conjectures, that the relax- 

 ation of the ciliary zone, so named by Zinn, 

 which appears to be nothing but the cap- 

 sule of the vitreous humour where it re- 

 ceives the impression of the ciliary pro- 

 cesses, permits the coats of the eye to push 

 forwards the crystalline and cornea. Such 

 a voluntary relaxation is wholly without ex- 

 ample in the animal economy, and were it 

 to take place, the coats of the eye would not 

 act as he imagines, nor could they so act un- 

 observed. The contraction of the ciliary 

 zone is equally inadequate and unnecessarj'. 



Some have supposed the pressure of the 

 external muscles, especially the two oblique 

 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 effectiug, no 

 sensible difference is produced in the dis- 

 tinctness of vision. 



Others say, that the muscles shorten the 

 axis : these have still less reason on their 

 side ; since such a change would lengthen 

 the focal distance, which in fact is longest 

 when the eye is at rest. 



Those who maintain, that the ciliary pro- 

 cesses flatten the crystalline, are ignorant of 

 their structure, and of the effect required : 



