. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, Fig. 109. 7. The Perforated Disc (Fig. 110).—This is a modifica-tion of the astigmatic chart. A piece of white cardboardor metal, about ten inches square, has small, round perfora-tions made in it of certain definite size. Each perforation isseparated from its neighbor by the distance of its openings are arranged in series of one, two, or threeparallel lines, exactly as in the pointed line test. Thischart or disc is hung on the window-pane, or an illumina
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, Fig. 109. 7. The Perforated Disc (Fig. 110).—This is a modifica-tion of the astigmatic chart. A piece of white cardboardor metal, about ten inches square, has small, round perfora-tions made in it of certain definite size. Each perforation isseparated from its neighbor by the distance of its openings are arranged in series of one, two, or threeparallel lines, exactly as in the pointed line test. Thischart or disc is hung on the window-pane, or an illumina-tion is placed behind it. The patient, looking at the disc, 140 REFRACTION AND HOW TO REFRACT. signifies which scries of perforations appear to coalesce andform hnes. This test is not commonly known or used. It. JiG. no. might be a valuable test if there was any convenient way of uniformly illuminating it from behind. 8. Prays Letters (Fig. iii).—These letters are of the Old Englishtype, and composed of strokes whichrun in different meridians. The pa-tient, looking at these letters, selectsthat letter which appears darker thanall the rest. The direction of the linesin the letter selected corresponds to themeridian of greatest ametropia. Thistest is very confusing to the patient,who sees first one letter and then an-other as darker than its Test.—This is an old test for ametropia, lloilH 55 iiiliiii o B ¥1 T a Fk;. Ill ASTIGMATISM. 141 good in theory, but rcall}- not sufficiently accurate for prac-tical purposes. It is explained for the students information,and not with the idea that he will ever take time to use test is made with a small piece of metal (Fig. 112)the size of the trial-lens, which contains two pin-point roundopenings at its center, separated by an
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