A handy-book of ophthalmic surgery for the use of practitioners.

  • Laurence, John Zachariah.
Date:
1869
Catalogue details

Licence: Public Domain Mark

Credit: A handy-book of ophthalmic surgery for the use of practitioners. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

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    L XII. XV. if, on the other hand, an eye to read I. has to approach to within half a foot from it, to read XII. three feet, to read XV. five feet, &c, then I. 2 XII. 4: =XV. 3 respectively. Or, if an eye at ten feet only reads XX., at five feet only XV., &c, then #=J and J respectively. S is con- siderably influenced by age. The following table exhibits the results of the investigation, by Dr. de Haan, of 281 cases of emmetropic eyes at different ages:— At 10 to 20 years S=o?\ 22 30 years = - 5o -S 14 fi0 =20 80 =1J: 20 The test-types of Professor E. Jager, of Vienna, consist of ordinary printers' letters ranging in size from No. 1 ( «« ) to No. 20 (u 8-line Roman). Unlike the test-types in Snellen's more scientific system, Jager's follow no other order than that of a gradual increase in the size of the letters.1 An eye with normal acuteness of vision should read Nos. 20 to 18 of Jager at 20 feet. A patient may be so amblyopic as not to be able to read CC (Snellen) at any distance. In such a case the ability to count fingers at different distances may be employed as a rough test of the patient's visual power. Or he may only be able to dis- tinguish light from darkness; even so low a visual power as this admits of differences. These may be tested by placing the patient some six feet from an Argand gas-flame in a dark room : 1 They possess, however, an advantage over Snellen's types in being those to which we are more accustomed in our ordinary reading.