An endeavour to show that insufficiency of the internal recti muscles and myopia have been erroneously associated : and that the muscular asthenopia of myopia is not the result of such insufficiency, but of the anomaly of refraction / by Samuel Theobald.

Theobald, Samuel, 1846-1930.

Date:: 1874

Credit: An endeavour to show that insufficiency of the internal recti muscles and myopia have been erroneously associated : and that the muscular asthenopia of myopia is not the result of such insufficiency, but of the anomaly of refraction / by Samuel Theobald. 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.

11/14 (page 11)

$In determining the strength of the prisms which are required in any given case, we should first ascertain the degree of the myopia, and then the character of the near work at which the eyes are usually employed, so that we may know for what distance the correction is to be made. If the myopia is found to be so great as to require the work to be performed nearer the eyes than would be otherwise desirable, we should, by combining a concave surface with the prism, remove the binocular far point to the desired distance; and we must, then, in our calculations, consider only the myopia which remains uncorrected. We should next ascertain the width between the centres of the pupils (which may be readily done, with sufficient accuracy, by means of an ordinary tape measure, or rule, held across the bridge of the nose, the patient being directed, in the mean time, to regard some distant object so that the optic axes may be parallel); and, having learned this, we may now find by calculation, in the manner before de- scribed, the actual excess of convergence over accommodation for the dis- tance which we have selected. This calculation, though not absolutely necessary in practice, is desirable because, as has already been pointed out, if the disproportion, thus obtained is found to be less than the apparent insufficiency shown by Yon Graefe’s test, it shows that besides the refractive anomaly, there are other causes tending to produce difficulty of convergence; and this knowledge would justify us in prescribing stronger prisms than we should otherwise do. By coiistructing a reference table which would show the disproportion proper to the different grades of myopia at the usual working distance, say 10°, with a maximum, medium, and minimum interocular width, we may save ourselves the trouble of making the calculation in each individual case. Most important, however, is it for our purpose, to carefully ascertain, by means of Yon Graefe’s test, the arp'parent insufficiency of the internal recti, at the distance for which it is proposed to make the correction ; be- cause it is mainly upon the result of this that our determination in regard to the strength of the prisms to be selected must depend. We should, moreover, not neglect to repeat this test after neutralization of the myopia by concave glasses; under which circumstances, if we are careful to observe that the patient accommodates accurately for the distance at which the test object is held, which he may be inclined not to do on account of the unusual effort required to overcome the action of the glasses, we shall generally find that all indications of insufficiency have disappeared. If, however, crossed diplopia still persists, and we are satisfied that accom- modation is being properly directed, we must conclude that there is either true insufficiency of the internal recti or difficulty of convergence, such as has been already referred to. And here I would suggest the advantage of substituting for the dot either a small cross or a star, as it will be found easier to accommodate accurately for them, owing to their more striking outlines. The line$