Handbook of optics for students of ophthalmology by Souter William Norwood 1861-

Author:Souter, William Norwood, 1861- [from old catalog]
Language: eng
Format: epub
Tags: Eye, Lenses
Publisher: New York, London, The Macmillan company
Published: 1899-03-25T05:00:00+00:00

changing the position of a lens in near vision, and we need not have deduced this result algebraically. We have seen that PQ (Fig. 32) is shorter when the lens, A^ is half way between P and Q than in any other position. Similarly, in Fig. 28, the least distance from F^ at which P may be situated and still form an image at this point is four times the focal length of the lens, and the lens must be at the midway point of the line PF^. Whether the lens be moved toward the object or toward the ej^e, the image will be thrown to the right of F^^ and the final image will be reduced in size. Thus the image attains its greatest size when the lens is in this midway position, and we have this rule for determining the effect of changing the position of the lens: As the convex lens is removed from the eye, the magnifying power increases so long as the distance of tlie object from the lens is more than twice tiie focal length of the lens, and when the distance between object and lens is less than twice the focal length of the lens, the magnifying power is diminished by further removal of the lens from the eye.

If, while the lens is convex, F^ — e be positive, that is, if the lens be placed within the anterior focus of the eye, then i^ will always be less than i.

We next suppose the lens to be concave. In this case F^ is negative. Our equation shows that when F^ is less than g, or when the lens is without the an-

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