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

  • Landolt, E. (Edmond), 1846-1926.
Date:
[1886]
Catalogue details

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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    by Nagelj and, also, in my own work.* In practice it is sufficiently accurate to divide the number of the prism by y to obtain, in metre angles, the deflection required to overcome it. (The distance between the centres of rotation of the two eyeballs in this case is taken as between 5S and 64 mm.) A prism of 140 held horizontally before one eye, requires from each a rotation of y = 2 m.a., adduction if the edge of the prism is turned inwards, abduction if outwards. Anyone who in distant vision can overcome a prism of 50 has a minimum of convergence — r = f = 71 m. a. When we desire to ascertain whether a patient's power of converging is sufficient or not, p should be taken as the starting point of our investigation. But in order to determine whether asthenopia is due to insufficiency of this function we must know, first of all, how much convergence the individual requires for his work. This important point is not easily ascertained, and, strange to say, seems to have been entirely overlooked. For vision at a distance of ~s 3 in. a. of convergence are essential. If, however, an individual's utmost capacity for convergence docs not exceed this amount; in other words, if his punctum proximum of convergence is situated at — continuous work at that distance will be impossible. For sustained effort a reserve of convergence power is necessary to replace what is expended in work. Only when we have gauged this reserve fund of strength can we tell with certainty whether the patient has motor- asthenopia or not, and if so, by how many metre angles he fails to reach the required standard. It is evident that this reserve power cannot be an absolute quantity (for instance, 2 m. a.) in all cases, but that it must represent a tolerably constant proportion of the total amount. My experience seems to show that this reserve force must be at least twice as great as the convergence employed. Thus in order to work at a distance of one metre, that is to