The student's guide to diseases of the eye . Fig. 10.—Foci of a convex lens. cf and c f are conjugate poifiis, and each is theconjugate focus of the other. The angle a or a re-maining the same, then if cf be further from the lensc f will approach it. A ray (c) converging to the axiswill be focussed at cf, because a = a; no real pointconjugate to c f exists ; but if the ray start fromc f it will, on taking the direction c, appear to comefrom vf, which is the virtual focus of c f (see § 6). 14. Concave lenses have only virtual foci. InFig. 11, a, parallel to the axis, is made divergent (see. Fig
The student's guide to diseases of the eye . Fig. 10.—Foci of a convex lens. cf and c f are conjugate poifiis, and each is theconjugate focus of the other. The angle a or a re-maining the same, then if cf be further from the lensc f will approach it. A ray (c) converging to the axiswill be focussed at cf, because a = a; no real pointconjugate to c f exists ; but if the ray start fromc f it will, on taking the direction c, appear to comefrom vf, which is the virtual focus of c f (see § 6). 14. Concave lenses have only virtual foci. InFig. 11, a, parallel to the axis, is made divergent (see. Fig. 11.—Foci of a concave lens. Fig. 7), its virtual focus being at/; similarly cf isthe virtual conjugate focus of the point emittingthe ray b. 15. In equally biconvex or biconcave lenses ofcrown glass the principal focus is at the centre ofcurvature of either surface of the lens. 16. Images.—The image formed by a lens consistsof foci, each of which corresponds to a point on the OPTICAL OUTLINES object. Given the foci of the boundary points of anobject, we have the position and size of its Fig. 12 the object a b lies beyond the focus /. Fig. 12.—Real inverted image formed by a convex lens. From the terminal point a take two rays a and a, theformer a secondary axis, and therefore unrefracted,the latter parallel to the principal axis, and thereforepassing after refraction through the principal focus /.These two rays (and all others which pass throughthe lens from the point a) will meet at a, the con-jugate focus of a. Similarly the focus of the poincb is found, and the
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