Text-book of ophthalmology . Micropsia and macropsia also occur in disorders of accommodation (see § 797) butare not then associated with distortion like that shown in Fig. 39.—D.] 117. Light Sense.—Let us assume that we have before us two personswho in ordinary daylight have the same visual acuity; both under equallygood illumination read print of the same size at the same distance. Wenow gradually lessen the illumination. As a result of this, the difference inbrightness between the black letters and the white paper diminishes and theletters arc distinguished with greater and greater difficul
Text-book of ophthalmology . Micropsia and macropsia also occur in disorders of accommodation (see § 797) butare not then associated with distortion like that shown in Fig. 39.—D.] 117. Light Sense.—Let us assume that we have before us two personswho in ordinary daylight have the same visual acuity; both under equallygood illumination read print of the same size at the same distance. Wenow gradually lessen the illumination. As a result of this, the difference inbrightness between the black letters and the white paper diminishes and theletters arc distinguished with greater and greater difficulty. At a certainstage in the process of obscuration, one of the two persons ceases to recog-nize the print while the other is still able to read, and the darkening has tobe carried furl her in order to make reading impossible for him. In this casewe say: The two persons have the same space sense, , the same suscep-tibility of the retina for impressions of forms, but they have a different light FUNCTIONAL TESTING 127. Fig. 40.—Photometer op Forster. sense (L)—, a different susceptibility for impressions of brightness andof differences of brightness. The light sense can be tested in various ways. We determine either the lowest limitof illumination with which an object is still visible (minimum stimulus) or the smallestdifference in brightness which can still be appreciated (minimum of differentiation). Themost usual method of measuring the light sense is with Forsters -photometer, which givesthe minimum stimulus. This instrument, which is represented in horizontal section inFig. 40, is placed in a perfectly dark room. A box, A, blackened on the inside, bears onits anterior wall two apertures for the two eyes, a and ah which look through these aper-tures at a plate, T, which is placedupon the posterior wall, and uponwhich large black stripes upon awhite ground are placed as test ob-jects. The illumination is pro-duced by a normal candle,3 L, thelight from which fa
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