Arabic test types. For the determination of visual acuity.

VISUAL acuity is determined by the smallest angle at which the eye is able to distinguish two points as separate. It is therefore inversely proportional to the minimum visual angle. The normal visual acuity is found to correspond to an average minimum visual angle of one minute. Thus with relatively small angles one may say that the visual acuity is inversely proportional to the space which separates two points or, when the space is constant, that it is proportional to the distance which separates the eye from the two points. However, the visual acuity cannot be determined by means of two points with a variable distance alone. This is because it is difficult to reckon precisely the exact moment when the two points cease to be seen separately and blend into a single visual impression. It is for this reason that for examining the acuity of vision the test objects must be slightly complicated. Acuity is therefore measured in terms of the visual angle subtended by the constituent parts of the test object. In 1862, Snellen introduced some scientific standardization into the measurement of visual acuity. His optotypes achieved such a wide and rapid success that most of the distance test types in clinical use to-day are modelled on Snellen's original chart. These optotypes are based on the following principle: Each letter in the Snellen chart-(Fig. 1) is designed in a square the sides of which are five times the width of the letter strokes. The breadth of the black strokes of the letter is equal to the white spaces between them and subtends an angle of one minute at the nodal point of the eye at the specified distance. Serifs are added to the larger strokes in order to maintain the unit separation throughout the letter. The whole letter subtends an angle of 5 minutes at the nodal point of the eye at the given distance.