On insufficiency of the power of convergence / by E. Landolt.

  • Edmund Landolt
Date:
[1886]
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Licence: Public Domain Mark

Credit: On insufficiency of the power of convergence / by E. Landolt. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    eyes to the object of fixation, whether the distance be finite, infinite, or even beyond infinity. The most natural method of measuring the convergence is by means of a fixed object placed in the median line (M M', Fig. I.) In this way the angle of convergence is always the same for each eye. This angle necessarily affords the measure of the convergence effort put forth by each eye. In distant vision, the two eyes (O and O') having a parallel direction, this angle is nil. It increases in proportion as the object fixed is brought nearer. It may be said then that the size of the angle of conver- gence is in inverse proportion to the distance between either eye and the fixed object in the median line.* If the object be at C at the distance O C=C, the angle of convergence J O C can be expressed by C= -^,. In measuring this distance C by the aid of the metre we obtain for the convergence required in binocular fixation an expression identical with that of the refraction necessary for distinct vision of the same object. Thus, supposing an object be situated at a distance of one metre from each eye there must be for both eyes ~ = I dioptry of positive refraction and I unit of positive convergence. This unit is called, after Nagel, to whom we are indebted for this principle of measurement, the metre angle A If the object is placed at \ metre from each eye = 3 D and 3 m. a. are required, and so on. The amplitude of convergence is obviously contained between the maximum and minimum of the convergence which an individual is capable of exerting. * In reality the inverse of the distance does not give the angle of conver- gence but its sine; for our purpose, however, we may conveniently substitute one for the other. t An expression analogous with metre lens, which Nagel has proposed for the dioptry. The absolute value of the metre angle depends upon the distance which separates the centres of rotation of the eyeballs from each other. For instance, if this, the base line, is 58 mm. in length, the metre angle=i°39'39. If 64 mm. the metre angle will = l°50'and soon.—Nagel in Graefe-Saemisch, Handbuch VI., p. 478.
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