This section is from the book "The Home Hand-Book of Domestic Hygiene and Rational Medicine. Volume 2.", by J. H. Kellogg, M.D.. Also available from Amazon: The Home Hand-Book of Domestic Hygiene and Rational Medicine, Volume 2.
In many instances, loss of distinct vision is so gradual that patients are scarcely aware of the fact until their sight has become very extensively impaired. This is especially the case when only one eye is affected. We have met a number of cases in which cataract had become fully developed without the individual being aware of the existence of any difficulty with the eye. Loss of vision is indicated whenever there is blurred sight of either eye with inability to read fine print or to see distinctly small objects which have once been readily discerned. The most accurate way of testing the sight is by means of "test types."
If an individual is unable to read under any circumstances the fine print known as "diamond," there is certainly some loss of sight. If he can read the finest type easily for a few seconds, but is then unable to read farther on account of the letters running together, the difficulty can probably be relieved by the use of proper glasses. In employing the test types, the distance at which the different varieties of type can naturally be read should bo observed. No. I should be easily read at a distance of one foot from the eye; No. II, at a distance of two feet; No. Ill, at three feet; No. VI, at four feet, and Nos. VII and XV, at seven and fifteen feet, respectively. Diamond type should be read at a distance of twenty inches from the eye. Pearl should be easily read at thirty, and minion at forty inches. When the letters or sentences can be easily read at the proper distance at first, but afterward cannot be made out without occasioning a tired feeling of the eyes, the indication is weakness of vision. When the test letters cannot be made out at any distance, there is almost entire loss of sight, probably the result of disease.
Test Types Eye Chart.