ABSTRACT
The standard of visual acuity is defined as the power to distinguish black objects on a white surface which subtend an angle of 5 minutes on the retina. A line of 1.45 mms. at a distance of 1 metre from the eye subtends this angle; and such a line can be distinguished at the distance named by the so-called ‘normal’ eye. When letters are used, as in Snellen's type sheets, the size of the letters is such as to produce an angle of 5′ vertically and horizontally, the strokes and spaces between the letters subtending an angle of 1′. In the diagram on the next page the lines are 2.9 mms. and therefore subtend an angle of 5′ at a distance of 2 metres, whilst the width of the lines subtends an angle of 1′.
