Involuntary Convergence, Eyestrain and Squint.

JOHANNESBURG IN ophthalmology there are two schools of thought, the physiological and the physical or mechanical school. Thus Williamson-Noble', dealing with my method of treating eyestrain, states: "curiously enough, Stutterheim designates the condition which his exercises are designed to improve as " involuntary convergence," whereas, as you will shortly see, it is the ability to perform the act voluntarily which is of real importance." He then describes how he has changed my technique of treatment into one with the rotary prism and the synoptophore, giving the " job " to the orthoptic trainer rather than doing it himself. This is the mechanical outlook. On the other hand Zeeman', collaborator of Brouwer3 at the Institute for Brain Physiology, in a press note on my work " Eye-strain and Conveigence "', states concerning my method: " It aims at harmonic training and widening of the important function of convergence, which is correctly c(alled " involuntary." By Elschnig5 convergence movements are called fusion movements. The " fusion movements" Elschnig refers to are the movements of convergence, as his description shows. He writes: " These movements are eye movements that alter the position of the eyes in such a way that the visual lines at every moment meet in the object of fixation. In normal binocular vision they occur unconsciously and involuntarily." (The italics are mine.) The range of these movements Elschnig gives as the sum of the strongest abducing prism and the strongest adducing prism that can be overcome by binocular fixation of a very far point without diplopia occurring. The normal abduction range (negative convergence) Elschnig gives as about 8 dioptres of prism, the normal adduction range (convergence) he gives as about 50 dioptres of prism. Duane6 measures a (voluntary) convergence, which he defines in this way: "The convergence power in a given case is best measured by determining the closest point to which a fine object accurately fixed by both eyes can be brought without either eye diverging." This is the so-called convergence test of, for instance, the R.A.F. It is simply a test for the muscular strength of voluntary squinting, or binocular adduction, very useful for that purpose, but by no means a test for real convergence, as the following figures will show. The range of normal convergence (or normal fusion movement according to the nomenclature of Elschnig (see also (4), p. 5), is 50 dioptres of prism. The equivalent …