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yet as the diffufion O S, occafioned by the central rays, is far lefs 

 than T V, that caufed by the exterior rays, vifion, though not 

 rendered perfedly diftind by the exclufion of the exterior rays, 

 will be made more clear than before, unlefs the denfity be very 

 much greater than it fhould, in which cafe the diffufion O S 

 might be fo great as to impede vifion entirely, as well as the diffu- 

 fion T V, which is greater than it. 



I HAVE here fuppofed the rays parallel, for the purpofe of 

 confidering the view of remote objedls ; but the reafoning will 

 equally apply to rays fenfibly diverging, and fo will illuflrate as 

 well the view of nearer objeds. 



These obfervations, if juflly founded, might ferve to fhew 

 the rcafon of what occurred in the fecond experiment, where the 

 confufion of vifion was fo much diminifhed by the contradion 

 of the pupil, the Iris in fuch a cafe intercepting thofe rays 

 which caufed a confufion when the pupil was dilated; in the 

 fame manner as the card in the firfl; experiment intercepted the 

 rays that caufed confufion to the naked eye. 



It appears to me that the defed of all fhort-fighted perfons 

 who are affifled by concave glaffes is probably of that kind repre- 

 fented in Fig. II. becaufe as the refradion of a lens afFeds any 

 rays that fall on it (except that in its axis) and as the refradion 

 is conflantly greater the more remote the incident rays are 

 from the axis, fo where fuch a glafs renders vifion diflind, 

 errors of refradion mufl have taken place in the eye, previous to 



the 



