File:Refraction and motility of the eye, with chapters on color blindness and the field of vision (1920) (14747193036).jpg

Identifier: refractionmotili00alge (find matches)
Title: Refraction and motility of the eye, with chapters on color blindness and the field of vision
Year: 1920 (1920s)
Authors: Alger, Ellice Murdoch, 1870- (from old catalog)
Subjects: Eye Eye
Publisher: Philadelphia, F. A. Davis petc., etc.)
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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The perimeter is the best instrument to measure de-fects in the periphery but its arc is so close to the eye thatsmall but important central defects which subtend a verysmall angle are easily overlooked and poorly recorded. Forthis reason the blackboard or better still the tangent cur-tain of Duane give much better tests, since the eye can beplaced 30 or 60 inches from its centre with a correspondingincrease in the scale of projection. The degrees marked onthe curtain are the equivalents of those on the perimeterat the distance chosen and can be charted in the same way. 340 REFRACTION AND MOTILITY OF THE EYE. About 15° outward from the fixation point a smalltest object falls on the projection of the optic nerve anddisappears in the so-called normal blind spot already spokenof. If the test object is moved in various directions in this
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Fig. 111. area while the patient calls out instantly when it comes intoview, the spot can be mapped out and marked very accu-rately. It seems somewhat larger when the object is movedfrom the blind into the seeing field than in the reversedirection, but should be a vertical oval about 7° by 5°. THE FIELD OF VISION. 341 Enlargement of the blind spot is a very important fact. Itmay be due to such manifest causes as a myopic crescentbut might also indicate such changes in the nerve as occurin early neuritis or optic atrophy or incipient glaucoma.On the other hand a normal blind spot would exclude manypathological conditions. Peters campimeter is a veryconvenient instrument for rapidly investigating the size ofthe blind spots and central defects (Fig. 111). For the discover)* and localization of very small defectsvery small test objects are necessary—used with great care. Changes in the field of vision are of two kinds: eitherthe boundary of the field is concentrically or irregularlyre

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