Defective eyesight, the principles of its relief by glasses.

  • Roosa, D. B. St. John (Daniel Bennett St. John), 1838-1908.
Date:
1899
Catalogue details

Licence: Public Domain Mark

Credit: Defective eyesight, the principles of its relief by glasses. Source: Wellcome Collection.

Provider: This material has been provided by the Harvey Cushing/John Hay Whitney Medical Library at Yale University, through the Medical Heritage Library. The original may be consulted at the Harvey Cushing/John Hay Whitney Medical Library at Yale University.

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    that of each of the twenty-five small quadrangles, of which each letter is formed. The numbers above them, CC, C, LXX, L, XXX, XX, indicate the distance in Parisian feet at which these letters can be seen at an angle of 5'. The visual power is then expressed by a formula, as follows: The visual power V, equals the distance at which the letters should be seen, divided by the dis- tance at which they are actually recognized by the patient. For example, if a patient sees letters that are normally distinguished at 20 feet, at only 10 feet, the formula would be V = i^. In the latter part of this book is found a specimen of Jaeger's and Snellen's test- types, which, with these remarks, will make this part of my subject perfectly clear. Although Snellen's test-types, like Jaeger's, include also a set of graduated type for testing reading power, the general practitioner, and, in many instances the specialist, will be sufficiently accurate, if he limit his collection of test-letters to Jaeger's for near vision, and Snellen's for the distance. If convenient, matters should be so arranged that the visual power may be recorded as found with Snellen's letters at a distance of 20 feet.