The student's guide to diseases of the eye / by Edward Nettleship.

  • Edward Nettleship
Date:
1882
Catalogue details

Licence: Public Domain Mark

Credit: The student's guide to diseases of the eye / by Edward Nettleship. Source: Wellcome Collection.

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    cidence. For thin lenses this is expressed by saying that the angle d, Fig. 5, is the same for the rays a a', hh', and cc', incident at different angles, but at the same distance from the axis. Fig. 5.—Refraction by a lens the same for all rays incident at same distance from axis. 8 An ordinary lens is a segment of a sphere (plano- convex or plano-concave), or of two spheres whose centres are joined by the axis of the lens (biconvex or biconcave). • n -i. 9. A lens is regarded as formed of an infinite number of minute prisms, each with a difterent re- fracting angle. Fig. 6 shows two such elements ot a Fig. 6.—Prismatic elements of a convex, lens, convex lens, in which the angle (a) of the pnsm at the edge of the lens is larger than the angle (^) of the prism nearer the axis. Hence of the two Parallel rays (a and h) a will (see § 5) be more ?efra ted than &, and the rays will after emergence conTerge and meet at/. Fig. 7 shows the correspond-
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