Page:Popular Science Monthly Volume 12.djvu/91

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EFFECTS OF STUDY ON THE EYESIGHT.

of instead of upon, the retina. This is near-sight, as it may he recognized by object-tests or trial-glasses. But near-sight is sometimes simulated. This is caused by a spasmodic action of the muscle of accommodation. To determine absolutely, therefore, whether or not the eyeball has taken this abnormal shape, or whether the apparent near-sight is due to this spasmodic action of the focalizing muscle, the oculist must paralyze that muscle. He does this by a simple and, in his hands, a harmless application of a weak solution of sulphate of atropia.[1] Then the object-tests and trial-glasses will determine the question with certainty. But, if it be impracticable to apply the atropia, then the ophthalmoscope[2] must be resorted to, as offering the nearest approach to certainty of results when the accommodating-muscle cannot be paralyzed, because its contraction is not very likely to occur under the operation of that instrument. Thus provided, the oculist proceeds to examine the interior of the eye, and, his own eye being normal, and his own accommodation relaxed, if he sees the retina of the examined eye perfectly, he pronounces the refraction to be correct; or, technically, the eye is emmetropic. But, if he finds the retina is not clearly visible, there being no opacity of the refracting media, he knows it can only be because the rays reflected from the ophthalmoscope have not converged upon it. Assuming it to be a case of anterior convergence, he interposes a concave glass, which lengthens the focus and removes the point of convergence back upon the retina. Thereupon he pronounces the eye near-sighted; or, technically, myopic, of a degree indicated by the strength of the glass.

Near-sight, then, is that condition of the eye in which the rays from distant objects reach the retina after convergence.

On the other hand, if, instead of the eyeball becoming elongated, it is flattened, then the visual axis is too short; that is, the retina is brought too near the lens, which consequently requires the contraction of the accommodating muscle to focalize the parallel rays upon the retina; whereas, had the eye been normal, the lens would have performed this function while in a state of rest, and would have required the contraction only for divergent rays.

  1. Though this is frequently done with individual patients, yet schools have generally objected to it. Dr. Cohn enjoyed an exceptional opportunity to examine the eyes of 240 scholars after the application of sulphate of atropia. Dr. Callan's colored subjects, he relates, refused to permit this application. Therefore, wishing "to place the results of his examination beyond dispute" in point of accuracy, he adopted the alternative course, and "kept both of his own eyes under the influence of a four-grain solution of sulphate of atropia, applied three times daily during a period of five weeks, so that the accommodation was completely paralyzed for that length of time." Sometimes the examining oculist has acquired the power to perfectly relax his accommodation at will. But the relaxation of the accommodation of the subject, as well as that of the examiner, is essential to entire accuracy.
  2. A small mirror with a hole in the centre. The mirror is held close to the patient's eye, so as to reflect into it the light of a gas-jet back of him. The oculist then places his eye close to the hole, and looks into the illuminated interior of the eyeball.