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the application of the glafs, equal and contrary to thofe refradions 

 whicli have removed them. ' 



If the defed were not of this nature, but that all the rays 

 were brought to a focus in the fame point before the retina, a 



Fig. III. lens, the refradion of whofe central parts would bring the rays 

 near the axis to a focus at the retina, will, by the greater 

 refradion of its exterior parts, remove the focus of the more 

 remote rays behind the retina. Or even though all the rays 

 did not meet in a point, yet if their interfedions were all very 



Fig. IV. clofe to each other, and at a confiderable diftance from the 

 retina, the efFed of a conclave glafs would be the fame as before, 

 and while it deftroyed the diffufion of the central rays, would 

 leave a diffufion of the exterior rays remaining, or vice vc-rfa. 



On the other hand, if the foci of the extreme and central 

 Fig. V. ""^y* within the eye fall before the retina, and are too diftant 

 from each other, no double concave of the ufual form (i. e. of 

 equal curvature on each fide) can deflroy the error of the one 

 kind of rays, without generating or leaving one of the other 

 kind. 



Nowr, if any glafs can be contrived in which the quantity of 

 refradion of the central rays can be made the fame as in a 

 double concave, while that of the exterior rays is diminifhed, 

 it would feem that the former error might be avoided j and on 

 the contrary, that the laft mentioned error might be compenfatcd, 

 if the glafs were fo contrived as to make the exterior rays 

 fuifcr a greater refradion than they do in the double concave, 



the 



