jr-o , ProltibU Ufa of the Cr;jilaUiw Humour. 



nor letters; but, upon making preflure with the fpcculum, he was enabled to dilliiiguini 



both the lines and the letters of the book with eafe. 



Such being this author's conception of the aftlon and effect of the external mufcles, he has 

 proceeded to apply the dodrines in explanation of the changes the eye is known to undergo 

 at different periods of life or habits of occupation, from their ilrength in early life, their 

 debility in old age, and their habitual aclion from ulV. 'ihi-fc and other general fads are 

 accounted for with confiderable addrefs. 



Mr. Home, the brother-in-law of Hunter, and vindicator c.f his pofthumous fame, was 

 appointed to reail the Croor.-m lecture on mufcular motion, for the feffion of the Royal 

 Society beginning in the year I794. Hisledlureis peculiarly valuable for-the expeiimental 

 rcfults it exhibits. In profeeuting the enquiry projected by Mr. Hunter, he had the great 

 advantage of the aflidance of his friend Mr. Rinifden, who, in converfing upon the diirercnl 

 ufes of the cryftalline humour, made the following obfervations : 



He faid that, as the cryftalline humour confifts of a fubftance of different denfitie.';, the 

 central parts being the moft compaft, and from thence diminifiiing in denfity gr.idually in 

 every direction, approaching the vitreous humour on one (Ide, and the aqueous humour on 

 the other, its refractive power becomes nearly the fame with that of the two contiguous 

 fubftanccs. That fome philofophers have (tated the ufe of the cryftalline humour to be, 

 for accommodating the eye to fee objtfts at different diftances; but the firmnefsof the cen- 

 tral part, and the very fmall diff.ience between its lefrattive power near the circumference, 

 and that of the vitreous or the aqueous humour, feemed to render it unfit for that purpofe; 

 its principal ufe rather appearing to be for correcting the aberration arifing from the fpheri- 

 cal figure of the cornea, v^here the principal part of the refra£tion takes place, producing 

 the fame effeit that, in an achromatic objedtglafs, we obtain in a lefs perfeft manner by 

 proportioning the radii of curvature of the different lenfes. In the eye the correction feems 

 perfeift, which in the objedl-glafs can only be an approximation ; the contr.iry aberrations 

 of the lenfes not having the fame ratio ; fo that, if this aberration be perfectly correfted, at 

 any given diflance from the centre, in every other it mult be in fome degree imperfc£t. 



Purfuing the fame comparifon: In the achromatic object-glafs we may conceive how 

 rnuch an object niufl appear'fainter from the great quantity of light loft by reflection at the 

 furfaces of the different lenfes, there being as many primary reflecflions as there are fur- 

 faces ; and it would be fortunate if this reflected liglit was totally loft. Part of it is again 

 rcflefted towards the eye by the interior furfaces of the lenfes; which, by diluting the image 

 formed in the focus of the objedt-glafs, makes that im.age appear far lefs bright than it 

 >%-ould otherwife have done, producing that milky appearance fo often complained of in 

 viewing lucid objects through this fort of telcfcope. 



In the eye, the fame properties that obviate this deff£t fcrve alfo to corrccTt the errors 

 from the fpherical figure, by a regular diminution of denfity, from the centre of the cryf- 

 talline outward. Every appearance (hews the cryftalline to confift of laminje of different 

 denfities ; and if we examine the junftion of different media, having a very fmall difference 

 of Tefra£tion, we fhall find that we may have a fenfible refraition without reflection. Now, 

 if the difference between the contiguous media in the eye, or the laminx in the cryftalline, 

 be very fmall, we Ihall have xefraCtion without having reflciftion ; and tliis appears to be the 



