OBSERVATION'S OK VISION. 



52T 



ble through the capsule, and sometimes the 

 anterior ones even through the cornea and 

 aqueous humour. When the crystalline falls, 

 . it very frequently separates as far as the cen- 

 tre into three portions, each having aline in 

 its middle. If it be carefully stripped of its 

 capsule, and the smart blast of a fine blow- 

 pipe be applied close to its surface in differ- 

 ent parts, it will be found to crack exactly in 

 the direction of the fibres above described, 

 and all these cracks will be stopped as soon 

 they reach either of the radiating lines. The 

 application of a little ink to the crystalline 

 is also of great use in showing the course of 

 the fibres. 



When first I observed the structure of the 

 crystalline, I was not aware that its muscula- 

 rity had ever been suspected. We have in- 

 deed seen, that Descartes supposed it to be of 

 a muscular nature ; he had, however, no 

 accurate idea of its internal structure. 



But the laborious and accurate Leeuwen- 

 hoek, by the help of his powerful micro- 

 scopes, has described the course of the fibres of 

 the crystalline, in a variety of animals ; and 

 lie has also called it a muscle* ; but, proba- 

 bly from, examining only dried preparations, 

 he has imagined, that each coat consists of 

 circumvolutions of a single fibre, and has in- 

 tirely overlooked the attachment of the fibres 

 to lines resembling tendons. If the fibres were 

 continued into each other in the manner that 

 he describes, the strict analogy to muscle 

 would be lost, and their contraction could not 

 conveniently have that eflfecton the figure of 

 the lens, which is produced by help of the 

 tendons. Yet much anatomical merit must 



• Now if the cristaline humour (which I have sometimes 

 called the crist. muscle) in our eyes, *c. Phil. Tram. 

 XXIV. IfiO'—CrystaUiimm musculum, alias humorem 

 cnjstallimim ttictum, 6fc, Leeuwenh. Op. omn. I. ]03. 



be allowed to the faithful description, and 

 elegant delineation, of the crystallines of 

 various animals, which he has given in 

 the Philosophical Transactions. (XIV. 780> 

 and XXIV. 1723). It appears, from hia 

 descriptions and figures, that the crys- 

 talline of hogs, dogs, and cats, resembles 

 what I have observed in oxen, sheep, and 

 horses ; that in hares and rabbits, the'' raK 

 diating lines on each side, instead of three, 

 are only two, meeting in the axis so as to 

 form one straight line; and that in whales 

 they are five, radiated in the same manner 

 as where there are three. It is evident that 

 this variety will make no material difference 

 in the action of the muscle. I have not yet 

 had an opportunity of examining the human 

 crystalline, but from its readily dividing into 

 tliree parts, we may infer that it is similar to 

 til at of the ox. The crystalline in fishes be- 

 ing nearly spherical, such a change as I atr 

 tribute to the lens in quadrupeds cannot take 

 place in that class of animals. 



It has been observed that the central part 

 of the crystalline becomes rigid by age, and 

 this is sufficient to account for piesby»- 

 opia, without any diminution ofthehumours ; 

 although 1 do not deny the existence of this 

 diminution, as. a concomitant circumstance. 



1 shall here beg leave to attempt the solu- 

 tion of some optical queries, which have not 

 beea much considered by authors. 



L Musschenbroek asks, What is the cause 

 of the lateral radiations which seem to adhere 

 to a candle viewed with winking eyes? I an- 

 swer, the most conspicuous radiations are 

 those which, diverging from below, form, 

 each with a vertical line, an angle of about 

 seven degrees ; this angle is equal to that 

 which the edges of the eyelids when closed 

 make with a horizontal line ; and the radia^- 



