Ocular refraction and the shadow test . s upon the surfaces A B and D B. By identical points it ismeant that one point is located upon one surface in a position corre- OCULAR R K F R A C T I O N. spending to the location of the other point upon the second ray R is refracted and if B C is extended as a straight line, Rwill meet B C at a point indicated as P; the ray S will also be refract-ed in a similar manner and its path intersects B C at P also. As a circle may be regarded as a regular polygon, that is, a planesurface bounded by straight lines of equal length, and of infinitenum
Ocular refraction and the shadow test . s upon the surfaces A B and D B. By identical points it ismeant that one point is located upon one surface in a position corre- OCULAR R K F R A C T I O N. spending to the location of the other point upon the second ray R is refracted and if B C is extended as a straight line, Rwill meet B C at a point indicated as P; the ray S will also be refract-ed in a similar manner and its path intersects B C at P also. As a circle may be regarded as a regular polygon, that is, a planesurface bounded by straight lines of equal length, and of infinitenumber; so a sphere may be regarded as a body bounded by an infi-nite number of equal plane surfaces. If a section be cut from a sphere so that the surface made by thecut be a plane, we have a body bounded by one plane and one-curvedsurface, every point upon the curved surface will be equally distantfrom the centre, (Figure 13), the curvature must therefore be the samein all meridians. If the spherical body be of glass, we have a refract-. 1 effects upon Figure 36. allel to base line B C of corresjxIt upon identical points E and F. ing medium whose power is alike in all meridians, as the relation ofthe two surfaces to each other is the same in all meridians. The surface made by the cut may be concave or convex as well asa plane, the three conditions are shown in figure 37. Consider thatthe surface A C D is made up of an infinite number of small planesI. 2, 3, 4, 5, 6, 7, etc; the refractive condition created is that of twopri^Tis of like measurements with their bases together. According tothe law of refraction and the experiment illustrated by figure 36, raysincident upon the surface A P D meet at a point P upon a line pas-sing through the points B C if the incident rays are parallel to thisline; the point P, where the parallel rays meet after refraction by acurved surfaces is called the principal focus. LENSES. 39 A transparent incdiicn having one curved and one plaiic surface, o
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