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The antients fuppofed that the cryftalline lens was the prin- 

 cipal feat of vifion, which they agreed to place in the centre of 

 the eye ; that the fpace between it and the bottom of the eye 

 was filled tJy the vitreous humour, and that the aqueous humour 

 occupied the anterior part of this organ. As the iris interfedted 

 this laft fpace, they agreed to call anterior^ or outer chamber of 

 the aqueous humour, the parts between it and the cornea trahf- 

 parens ; znd pojierior, or inner chamber, what remained between 

 the cryftalline lens and it. The catarad, it was affirmed, was a 

 web or membrane formed immediately behind the pupilla, in this 

 pofterior chamber, and far removed from the cryftalline, not un- 

 like a fcum that is fometimes found on the top of bottled liquors 

 not well corked. But as experience proved that people after the 

 removal of this opaque body by no means faw with that diftind- 

 nefs that might be expeded, and that theory and pradicc might 

 go hand in hand, this phsenomenon was accounted for by ob- 

 ferving — " That in the formation of this fcum or membrane. 

 " the moft denfe parts of the aqueous humour were engaged, 

 •' the remainder of this liquor was therefore rarer, or lefs enabled 

 " to caufe a convergence of the rays of light, and fight muft of 

 " courfe be proportionably weaker." 



Towards the decline of the laft, but particularly fince the 

 commencement of the prefent century, repeated difledions and 

 obfervations made it but too evident that the cataraB was not a 

 membrane^ but the cryflalline lens itfelf^ that was retidered opaque. 

 Numbers of cafes, and many works were publifhed from time 

 to time to corroborate this fid, which were violently oppofed 

 by the partizans of the former dodrine ; the chief of their argu- 

 ments, 



