Explanatory text to Wood's ophthalmic test-types and color-blindness tests / by G. R. Cutter.

  • Cutter, George R. (George Rogers), 1840-
Date:
1880
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Licence: Public Domain Mark

Credit: Explanatory text to Wood's ophthalmic test-types and color-blindness tests / by G. R. Cutter. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    We do not mean by this that the diagnosis is always very easy. Prac- tice and knowledge are necessary. Moreover, there is a long series of degrees from incomplete color-blindness to normal vision on the one side, and, on the other, to complete color-blindness. There must naturally be a border-line where differences of the two kinds of color-blindness cease to be recognized. The examination may end with this test, and the diagnosis be consid- ered as perfectly settled. It is not even necessary, practically, to decide whether the color-blindness is red or green. But to be more entirely convinced of the relation of complete color-blindness with the’ signal colors, and especially to convince, if necessary, the railway employees and others who are not specialists, the examination may be completed by one more trial. The one we are going to mention is not necessary to the diagnosis, and only serves to corroborate the investigation. Test III.—The red skein is presented to the subject. It is necessary to have a vivid red color, like the red flag used as a signal on railways. The color should be that of II. h of the plate, rather towards yellowish red. Rule.—This test, which is applied only to those completely color-blind, should be continued until the person examined has placed beside the specimen all the skeins belonging to this shade, or the gi-eater part, or else, separately, one of several colors of confusion (10—13). The red- blind then chooses, besides the red, green and brown shades, which (10—11), to the normal sense, seem darker than red. On the other hand, the green-blind selects opposite shades, which appear lighter than red (12—13). Remark.—Every case of complete color-blindness discovered does not always make the precise mistakes we have just mentioned in the preced- ing examinations. These exceptions are either instances of persons with a comparatively inferior degree of complete color-blindness, or of color- blind persons who have been exercised in the colors of signals, and who endeavor not to be discovered; they therefore usually confound at least green and brown, but even this does not always happen. Additional note.—We have not given rules for discovering total color- blindness, because we have not found any cases of this kind. If any such should be found, they will be recognized, according to the theory, by a confusion of every shade having the same intensity of light. Violet-blindness will be recognized by a genuine confusion of purple, red, and orange in the second test. The diagnosis should be made with discrimination. The first test often shows blue to bea colorof confu- sion; this may, in certain cases, be the sign of violet-blindness, but not always. We have not thought it advisable to admit defects of this kind, only the most marked cases, tliat other examinations establisli as violet color-blindness, should be reckoned in the statistics. Finally, to acquire a desirable uniformity, it is necessary to add, that, in the preparatory examination, it is my habit to indicate in the journal, especially kept for that purpose, cases of complete color-blindness by 2 (2 R; 2 G; 2 V), those of incomplete blindness by 1, and those of feeble chromatic sense by 0.5 (0.5 R, 0.5 G; 0.5 V).
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