Text-book of ophthalmology . Fig. 369.—Varying Size of the Diffusion Circles According to the Distance from the Retinaof the Focus of the Rays. less near-sighted with increasing years because they see better at a dis-tance; but this is often simply due to the circumstance that their pupilsdiminish in size with age. Far-sighted persons who are compelled toread close to them without convex glasses try to get as brilliant an illumi-nation as possible, so that their pupils may become very greatly contractedand thus diminish the size of the diffusion circles. The same object issecured to a still gr
Text-book of ophthalmology . Fig. 369.—Varying Size of the Diffusion Circles According to the Distance from the Retinaof the Focus of the Rays. less near-sighted with increasing years because they see better at a dis-tance; but this is often simply due to the circumstance that their pupilsdiminish in size with age. Far-sighted persons who are compelled toread close to them without convex glasses try to get as brilliant an illumi-nation as possible, so that their pupils may become very greatly contractedand thus diminish the size of the diffusion circles. The same object issecured to a still greater extent by placing a fine stenopeic aperture beforethe eye. This allows only a very narrow beam of rays to pass, and reduces. Fig 370.—Varying Size of the Diffusion Circles with Varying Width of the Pupil. the diffusion circles so greatly that they no longer exert a disturbing we repeat the experiment made above of attempting to look simultane-ously at the book and the pencil point, and while we are doing it hold aminute aperture before the eye, we see the print and the pencil pointdistinctly at the same time. By means of a stenopeic aperture myopicpersons can see distinctly at a distance even without concave glasses. The pupil, being the base of the cone of light, determines not onlythe size but also the shape of the diffusion circle, which accurately mirrorsthe shape of the pupil. In this way it happens that persons with irregular OPTICAL PROPERTIES OF THE NORMAL EYE 851 pupils (owing, for example, to posterior synechias) are very well able them-selves to perceive these irregularities entoptically. 758. Accommodation and its Mechanism.—In making the above ex-periment with the pencil and book,
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