Appendix II.

397

hot wonder, that if by so great a Quantityof Light which is thrown upon old Eyesby very convex Glasses, they should becomedebilitated, weak, and watery in the Courseof a few Years. So potent a Cause asthat of Light by the Constancy of its Ope-ration produces very considerable Effects,though by very flow Degrees. The con-stant Dripping of Water will in a Course ofTime excavate the hardest Stone,* though itsimmediate Effects be not in the least dis-cernible, nor any Thing more conlideredthan the Action of Light upon the Eye.

The Difference then between the Vi-sual Glasses is as represented in Figure 4,where A B is the open Glass, as in com-mon Spectacles, and C D the Glass of areduced Size in the visual Form ; the darkstart between that and the Steel FrameE F is a Black Zone or Circle of Horn inwhich the Glass is placed, and serves at thefame Time as a Safe-guard and Defenceto the Eye against extraneous Light.

The second Thing in which the com-mon Spectacles are most egregioufly faulty,is their being placed direct before the Eye,or both in the fame Plane, by which Means’the Axes of the Glass and those of theEye make a considerable Angle with each

other;

+.