The Vision of the Pupils of an Elementary School Tested by the Alphabet and Illiterate Cards

In 1862 Dr. Herman Snellen, professor of ophthalmology in the University of Utrecht, Holland, and director of the Netherlands Institute for Diseases of the Eye, published an improved series of test types for measuring the acuity of vision. The principle which guided him in the construction of these test letters is based upon the fact that the normal eye can just discern objects that subtend a one minute angle, the vertex of the angle being the point where the rays of light cross before falling on the retina. In order that a letter may be recognized, each one of its elements must be discernible, and hence each of these elements must have a diameter equal to the tangent of an angle of at least one minute. In constructing uniform letters in conformity to this principle, Snellen found that each letter must have at least one diameter equal to the tangent of a five minute angle. Each letter is therefore made in a square which is subdivided into twenty-five equal squares, each small square being equal in diameter to the tangent of a one minute angle. Since the tangent of a five minute angle is equal to 0.001454, to obtain the longest diameter of a letter to be seen at a given distance, Snellen multiplied the length of the tangent of five minutes by the distance (in centimeters) of the letter from the nodal point of the eye; thus, at a distance of one hundred centimeters the height of the letters should be 0.1454 centimeter, and each stroke of the letter should be at least one-fifth of this, or 0.0291 centi-