On the Mechamfm cf the Eye. ^2^ 



axis, be at all equivalent to the increafe of curvature : we mud therefore fuppofe fome 

 different mode of aftion in the power producing the change. Now, whether we call the 

 lens a mufcle or not, it feems demonflrable, that fuch a change of figure takes place as can 

 be produced by no external caufe ; and we may at leaft illuftrate it by a comparifon with' 

 the ufual action of mufcular fibres. A mufcle never contradls, without at the fame time' 

 fwelling laterally, and it is of no confequence which of the effeQs we confider as primary. 

 I was induced, by an occafional opacity, to give the name of membranous tendons to the 

 radiations from the centre of the lens ; but, on a more accurate examination, nothing 

 really analogous to tendon can be difcovered. And, if it were fuppofcd that the parts neUt 

 the axis were throughout of a tendinous, and therefore unchangeaWe nature, the contrac- 

 tion muft be principally effected by the lateral parts of the fibres ; fo that the coats would 

 become thicker towards the margin, by their contraflion, while the general alteration of 

 form would require them to be thinner j and there would be a contrariety in the anions of 

 tlie various parts. But, if we compare the central parts of each furface to the belly of the 

 mufcle, there is no difficulty in conceiving their thicknefs to be immediately increafed, and 

 to produce an immediate elongation of the axis, and an increafe of the central curvature ; 

 while the lateral parts co-operate more or lefs, according to cheir diflance from the cent'l'fr, 

 and in different individuals in fomewhat different proportions. On this fuppofition, we 

 have no longer any difficulty in artributing a power of change to the cryftalline of fiflies. 

 M, Petit, in a great number of obfervations, uniformly found the lens ef fiflies more or 

 lefs flattened : but, even if it were not, a flight extenfion of the lateral part of the fuperfi- 

 cial fibres would allow thofe fofter coats to become thicker at each vertex, and to form the 

 whole lens into a fpheroid fomewhat oblong ; and here, the lens being the only agent in 

 refraflion, a lefs alteration than in other animals would be fufficient. It is alfo worthy of 

 inquiry, whether the fliate of contradlion may not immediately add to the refraSive power. 

 According to the old experiment, by which Dr. Goddard attempted to (how that mufcles 

 become more denfe as they contraft, fuch an effect might naturally be expefted. That 

 experiment is, however, very indecifive, and the opinion is indeed generally exploded, 

 but perhaps too haftily ; and whoever fhall afcertain the exiftence or non-exiftence of fuch 

 a condenfation, will render efl^ntial fervice to phyfiology in general. 



Dr. Pemberton, in the year 1719, firft fyftematically difcuffed the opinion of the muf- 

 cularity of the cryftalline lens *. He referred to Leeuwenhoek's microfcbpical obferva- 

 tions ; but he fo overwhelmed his fubjcfl with intricate calculations, that few have at- 

 tempted to develope it : and he grounded the whole on an experiment borrowed from 

 Barrow, which with me has totally failed ; and I cannot but agree with Dr. Olbers in the 

 remark, that it is eafier to confute him than to underftand him. He argued for a partial 

 change of the figure of the lens y and perhaps the opinion was more juft than the reafons 



• De Facttkate Oculi q^ua ad diverfas Rerura diftantias fe accommodat. L. B. 1719. Ap. Hall. Difp. 

 Anat.IV. p. 301. 



Vol. v.— December 180J. Uu adduced. 



