Atlas of the external diseases of the eye : including a brief treatise on the pathology and treatment / by O. Haab ; authorized translation from the German ; edited by G.E. de Schweinitz.

Haab, O. (Otto), 1850-1931.

Date:: 1903

Credit: Atlas of the external diseases of the eye : including a brief treatise on the pathology and treatment / by O. Haab ; authorized translation from the German ; edited by G.E. de Schweinitz. Source: Wellcome Collection.

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$the distance, or 10 nieters. Tliis row of letters is desig- nated by the mimber 10, which is their normal distance. If no'smaller letters than these can be discerned, yision is 4, or, keeping the same fraction, /y. The letters in the third row are three times as large as those in the first row (which are 7.5 mm. high); a normal eye should therefore be able to read them at three times the distance, or 15 meters. Tliis row is mark cd 15, its normal distance. If an individual cannot read any let- ters smaller than these, Ins vision is evidcntly or, keep- ing the same fraction, yb. The letters in the fourth row are four times as large as those in the first row, and are designated by their normal distance, 20 meters. If these letters, which a normal eye can read at four times the distance, or 20 meters, are the smallest that can be discerned, vision is evidently or yy. The letters in the fifth row are six times as large as those in the first row, and above them is a single large letter, teil times as large as the first, which correspond respectively to visions of 4 (or -Zy) and Ä (or yb). The normal distance at which the sixth row should be read is 6x5, or 30, and it is accordingly marked 30; similarly, the large, single letter at the top is marked 50. The large- letter test is equivalent to the finger-test. Now we can measure visual aeuities ranging from yb (or 4) to -b (or yb) without changing the position of the type-card, which remains fixed at a distance of 5 meters. Or, the type-card may be fixed at a distance of 10 meters, in which case.the rcadings would be -]-{■- 1, \ ■§•, t|} 1, according to the letters the patient is able to read. It appears therefore that the numerator in the fraction corresponds to the distance in meters of the patient from the type-card, and (he denominator corresponds to the distance at which the type should be read normally. Examplc : The distance of the patient from the card is 5 meters; if the type marked 15 is discerned the vision is yy, or y. A simpler way of stating the rule is: Above the line$