A text-book of physiology, for medical students and physicians . ts within the limits specified will have their correspondingconjugate foci, at winch their images will be formed by the , if a luminous point is placed at v, Fig. 123, nearer to thelens than the principal focal distance, the cone of strongly di-vergent rays that falls upon the lens, although retracted, is stilldivergent after leaving the lens on the other side and consequentlyis not focused and forms no real image of the point. For every lensthere is a point known as the optical center, and for biconvex lensesthis poin
A text-book of physiology, for medical students and physicians . ts within the limits specified will have their correspondingconjugate foci, at winch their images will be formed by the , if a luminous point is placed at v, Fig. 123, nearer to thelens than the principal focal distance, the cone of strongly di-vergent rays that falls upon the lens, although retracted, is stilldivergent after leaving the lens on the other side and consequentlyis not focused and forms no real image of the point. For every lensthere is a point known as the optical center, and for biconvex lensesthis point lies within the lens, o. The line joining this center andthe principal focus is the principal axis of the lens {p-F, Fig. 123).All other straight lines passing through the optical center are knownas secondary axes. Rays of light that are coincident with any of thesesecondary axes suffer no angular deviation in passing through thelens; they emerge parallel to their line of entrance and practicallyunchanged in direction. Moreover, any luminous point not on the. I ig. I _I -Diagram illu frate the formation of an image by a biconvex lens: o, For-mation of the image of a point ; /;, formation of the images of a series of points. principal axis will have its image (conjugate focus) formed some-where upon the secondary axis drawn from this point through theoptical center. The exact position of Hie image of such a pointcan be determined by the following construction (Fig. 121): the luminous poinl in question. It will throw a cone ofrays upon the lens, the limiting rays of which may be represented byA-l) and A-c. One of these rays, .1-/, will be parallel to the prin-cipal axi . and will therefore pass through the principal focus, /. \i the di tance 18 determined and is indicated properly in the construction, the line .1-// may be drawn, as indicated, so ;is tothrough F after leaving the lens. The point- at, which the DIOPTRICS OF THE EYE. 301 prolongation of this li
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