3o8 IFhethtr the focal Adjujlmini of the Eye 



HI, HK, the uvea ; its anterior furface the iris ; its pofterior furface lined with pigtnentum 

 nigrum; IK, the pupil ; HL, HL, tlie cilinry proceflcs covered with pij^nienium nigrum ; 

 MM, the retina; N, the atjuvous liumcur; O, the crylbJliiic lens; P, the vitreous humour; 

 QR, QR, the zona ciliaris ; RS, RS, the annulus niucofus. 



Fig. 2. The ftru£lure of the cryilalHnc kns, as viewed in front, lil;cwife of the naturat 

 fize ; — Fig. 3. a fuie view of the cryllallinc. 



In the following year, 1794, a communication was made to the Royal Society, by EverarJ 

 Home, Efq. F. R. S. of fome fafls relative to a preparation for the Croonian Ledure, by 

 the late Mr. John Hunter. He ftates that this celebrated anatoniift had for many years 

 entertained the notion, tliat the cryllallinc humour was enabled by its own internal actions 

 to adjuft itfelf fo as to adapt the eye to different diftanccs ; and when the tarnia hyd.itigcna 

 firft came under his obfervation as a living animal, he was furprifcd to fee the quantity of 

 contraction that took place in a membrane devoid of mufcular fibres ; bur made ufe of the 

 fa£t in liis invcfligation of the fl.ru£lure of the cryftalline humour of the eye. Some 

 time after this, he difcovered tlie fibrous flruflure of the cryftaUinc humour in the eye 

 of the cuttle-filh, in which it is peculiarly dillimSl ; and thence he was led to confidcr the 

 exieiior part of this humour as fimilar in all animals. It was his intention to have afcer- 

 tained by experiment, whether mufcular adiicn does in faft take place ; and having found 

 that a certain degree of heat applied through the medium of water will excite mufcular adlion, 

 after almolt every other ftiniulus had failed, it was propofed to apply this to the cryftalline 

 humour, and afcertain its effeills. The cryftalline humour taken from animals recently killed 

 muft be confidered as being ftill alive. Such humours were to be immerfeil in water of 

 different temperatures, and placed in fuch a manner as to form the image of a lucid, well- 

 defined objeft, by a proper apparatus for that purpofe, fo that any change of the place of 

 that image, from the ftimulating effe£Vs of the warm water upon the humour, would be 

 readily alcertained. Thefe were the experiments wliich IVIr. Hunter had inftituted and- 

 begun, but in which he had not made fufficient progrefs before his death to enable him ta- 

 draw any concluGons. 



This communication contains an unfiniflied letter from Mr. Hunter to Sir Jofeph Banks, 

 containing fome prefatory obfervations leading to the ftatcment made by Mr. Home. It 

 appears, in faO, as that gentleman remarks, that the difcovery of a fibrou^appcarance in 

 the cryftalline appertains to Lewenhoek ; but that the difcovery of an eye in which the 

 ftrufture is ur»ccmmonly diftin<3, is due to Mr. Hunter. 



To this paper is annexed a plate exhibiting two fe£lions of the cryftalline humour of the 

 cuttle- fifti. 



In the fecond part of the Tranfaftions for the fame yeaf, I find a paper of fome length on 

 vifion, by Dr. Hoflack. This author fliews in the firft p.ace, that the enlargement or con- 

 tiaftion of the ptipil of the eye is infuflicicnt to produce the adjuftment by which the rays of 

 light fhall converge to a point in the retina ; and in fail a flight obfervation of what hap-, 

 peas is fnfticient to ftiew that the variations of the pupil are governed by different circum- 

 i\ances. He controverts Mr Young's deduftions with regard to the mufcularity of the 

 cryftalline, and even difputes the fafts. He quotes fome authorities to ftiew that the eye 

 U capable of accommodating kfelf to different diftances without the afiiftance of the cryf- 



3 talline. 



