[ ^9 ] 



That fuch phasnomena however may refult from various dc- 

 feds in the ftrudure of the eye, will be evident to any one 

 who confiders the exad conformation and adjuftment of its parts 

 neceffary to produce perfedt vifion. Thefe rcquifitcs indeed arc 

 fo many, and of fo fubtile a nature, that it feems much more 

 furprizing that fo delicate an organ fhould not be oftener rendered 

 imperfed by accident or natural infirmity, than that fo many 

 inftances of defedivc fight fhould be met with. 



A DEVIATION from the juft form or refradive power of any of 

 the different humours might be lliown (from optical principles) 

 capable of caufing the phasnomena above-mentioned ; but as the 

 chryftalline humour is the principal inftrument in vifion, it will 

 be fufficient to point out the cffeds arifing from any deficiency in 

 its ftrudure, efpecially as the number and minutenefs of the cir- 

 cum'ftances neceffary to its perfedion make this humour more 

 liable to incur the defed than either of the others; and as any 

 remedy applicable to the removal of errors hence arifing will be 

 found equally fo to that of errors occafioned by the imperfedion 

 of the aqueous or vitreous humours. 



The chryftalline humour being in the form of a double convex 

 lens, it is evident that when parallel rays are incident on it, 

 thofe fall more obliquely on its furface which are more remote 

 from the vertex or middle point of that furface. Now if that 

 humour were of uniform denfity, the refradion of the remote 

 rays would fo far exceed that of the central ones as to interfed 

 each other nearer to the chryftalline than the mutual interfedion 

 of the more central rays ; therefore if one of thefe interfedions 



(or 



