408 VHILOSOPHiCAL TRANSACTIONS. [aNNO 1794. 



probable, that on removing it, the place which it occupied is again filled by the 

 vitreous humour, whose power of refraction is nearly equal. At the same time 

 we cannot suppose the lens an unnecessary organ in the eye, for nature produces 

 nothing in vain; but that it is not of that indispensable importance writers on 

 optics have taught us to believe. 



4thly, Mr. Young tells us he has not yet had an opportunity of examining 

 the human crystalline; and grants, that from the spherical form of it in the 

 fish, such a change as he attributes to the lens in quadrupeds cannot take place 

 in that class of animals. The lenses which I have examined in the manner 

 above-mentioned, were the human, those of the ox, the sheep, the rabbit, and 

 the fish, and in all the same lamellated structure is observable; even in the sphe- 

 rical lens of the fish these lamellae are equally distinct, but without the smallest 

 appearance of a muscle. 



From these circumstances I cannot avoid the conclusion, that they do not 

 exist; at the same time I am persuaded that Mr. Young met with appearances 

 which he supposed were muscles; but I am satisfied he will readily acknowledge, 

 that the examination of the crystalline lens in its viscid glutinous state, is not 

 only attended with much difficulty, but that the smallest change of circumstan- 

 ces might lead to error; which I apprehend may probably have been the case in 

 that instance. On examining it after boiling, or exposing it to a gradual degree 

 of heat before the fire, when it may be handled with freedom, he will readily 

 observe, without a glass, the numerous lamellae, and the arrangement of their 

 fibres, which I have described. 



Another opinion has been sanctioned by many respectable writers, of the 

 effects of the ciliary processes in changing the shape and situation of the lens; 

 some supposed it to possess the power of changing the figure of the crystalline, 

 rendering it more or less convex*; others, that it removed it nearer to the cor- 

 nea-}-; and others, that it removed it nearer the retina;]]. The advocates for 

 these different opinions all agree in attributing these efiects to a supposed muscu- 

 larity of the ciliary processes. Of the structure of these processes Haller ob- 

 serves, ' In omni certe animalium genere processus ciliares absque uUa musculosa 

 ' sunt fabrica, mere vasculosi vasculis serpentinis percursi molli facti membrana.' 

 Which structure I believe at present is universally admitted. But even sup- 

 posing them muscular, such is their delicacy of structure, their attachment and 

 direction, that we cannot possibly conceive them adequate to the effects ascribed 

 to them. Besides, what we observed of the muscles of the lens itself, also ap- 

 plies to the processes, viz. that they may be destroyed, as in couching or ex- 



* Des Cartes, Scheinerus, Bidlous, Mollinettusj Sanctorius, Jurin. 



+ Kepler, Zinn, Portertield. 



J La Chariere, Perrault, Hartsoeker, Brisseau, and Derham. — Orig. 



