AJjuJlment of the Es/e deprived of the Cr^'falline. 3 1 1 



ftate of the eye ; for although we have two furfaces of the aqueous, two of the cryftalline, 

 and two of the vitreous humour, yet we have only one refiefted image ; and that being from 

 the anterior furface of the cornea, there can be no furface to refleft it back, and dilute an 

 image on the retina. 



This hypothcfis may be put to the ted whenever accident flial! furnifh us with a fubjeiS 

 having the cryftalline extradleJ from one eye, the ether remaining perfeft in its natural 

 ftate ; at the fame time we may afcertain whether or no the cryftalHne is that part of the 

 organ which ferves for viewing objefts at different diftances diftindly. Seeing no re- 

 flection at the furface of the cryftalline, might lead fome perfons to infer that its refraflive 

 power is very inconfiJerable ; but many cireumftances fliew the contrary : yet what it 

 really is may be readily afccrtained by having the focal length and diftance of a lens from 

 the operated eye, that enables it to fee objefts the moft diftinftly ; alfo the focal length of 

 a lens, and its diftance from the perfeft eye, that enables it to fee obje£ls at the fame dif- 

 tance as the imperfed eye : thefe data will be fufhcient whereby to calculate the refraftive 

 pow^r of the cryftalline with confiderable precifion. 



Again, having the fpherical aberration of the different humours of the eye, and having 

 afcertained the refraftive power of the cryftalline, we have data from whence to determine 

 the proportional increafe of its denfity as it approaches the central part, on a fuppofition 

 that this property corre£ls the aberration. 



An opportunity prefented iffelf for bringing the obfervations of Mr. Ramfden refpeding 

 the ufe of the cryftalline lens, to the proof. A young man came intoSt. George's Hofpital, 

 with a cataraft in the right eye. The cryftalline lens was readily extraded, and the union 

 of 'the wound in the cornea took place unattended by inflammation, fo that the eye fuf- 

 fered the fmalleft degree of injury that can attend fo fevere an operation. The man himfelf 

 was in health, 21 years of age, intelligent, and his left eye perfedV : the other had been an 

 uncommonly ftiort time in a difeafed ftate, and 27 days after the operation appeared to be 

 free from every other defe£l but the lofs of the cryftalline lens. 



A number of experiments were made on the imperfedl eye, alTifted by r. lens, and com- 

 pared with the perfeft eye. The aim of thefe trials, which wercjudicioully varied, was to 

 afcertain whether the eye which had been deprived of the cryftalline lens was capable of ad- 

 jufting itfelf to diftinct vifion at different diftances. Among other refults, the perfe£l eye, 

 with a glafs of 63 inches focus, had diftinft vifion at three inches ; the near limit was 

 1 J inch, the diftant limit lefs than 7 inches. The impcrfefl eye, with a- glafe 2-rVths 

 inches focus, with an aperture -^'^ths of an inch, had diftinft vifion at 2^ inches, the near 

 limit i|- inch, and the diftant limit 7 inches. The accuracy with which the eye was 

 brought to the fame point, on repeating the experiments, proved it to be uncommonly corredl; 

 and as he did not himfelf fee the fcale ufed for admeafurement, there could be no fource of 

 fallacy. From the refult of this experiment it appears that the range of adjuftment of the 

 imperfcfl eye, when the two eyes were made to fee at nearly the fame focal diftance, exceeded' 

 that of the pcrfeft eye. Mr. Ramfden fuggcftcd a reafon why the point of diftind vifion 

 of the impcrfeft eye might appear to the man himfelf nearer than it was in reality; namely, 

 that from the imperfedion of this organ, he might find it cnfier to read the letters wheii' 

 they fubteiidtd a greater angle than at his real point of diftinft vifion. The experiments, 

 however, appear to fliew that the internal power of the eye, by which it Is adjuftcd to fee at 



diflercnt 



