Refraction and muscular imbalance, as simplified through the use of the ski-optometer

by Daniel Woolf

Adduction

Adduction, or relative convergence, is the power of the internal muscles to turn the eyes inward; prism power base out and apex in, is employed.

Fig. 22—To test adduction, base out is required. Rotary prism’s line or indicator should be rotated from zero outwardly.

To test abduction, base in is required. Indicator should be rotated inwardly from zero.

To test adduction of the patient’s right eye, the rotary prism should be placed in position before the right eye, the red line or prism indicator being registered at zero upon the prism upper scale. The two cyphers (0) should be placed in a vertical position with the handle pointed horizontally ([Fig. 21]). The rotary prism should then be rotated so that its red line or indicator is rotated outward from zero until the large letter—preferably the largest letter, which is usually “E”—on the distance test-type or the Greek cross previously referred to, first appears to double in the horizontal plane. The reading on the scale of measurements should accordingly be noted. This test should be repeated several times, constantly striving for the highest prism power that the patient will accept without producing diplopia. The prism equivalent thus obtained will indicate the right adduction and should be so recorded, as designated in [Fig. 24]. The amount of adduction ranges from 6 to 28, prism diopters, the normal average being 24.