The purpose of the present chapter is to acquaint the refractionist with the operation of the Ski-optometer as “a scientific instrument for muscle testing”—the subject being treated as briefly and comprehensively as is practicable. As the reader progresses in the subject of muscular anomalies, he may carry his work to as high a plane as desired, increasing his professional usefulness to an enviable degree. Through the use of the Ski-optometer, muscle testing may be accurately accomplished in less time than a description of the operation requires. Furthermore, tedious examinations may be wholly overcome through the discontinuance of the consecutive transference of the various degrees of prisms from the trial-case. In fact, the latter method has long been quite obsolete, owing to the possibility of inaccuracy. The muscle action of the eye is usually quicker than the result sought through the use of trial-case prisms; hence muscle testing with the Ski-optometer is accomplished with far greater rapidity and accuracy, thus making the instrument an invaluable appliance in every examination. THE ACTION OF PRISMS Students in refraction—and one may still be a student after years of refracting—are sometimes puzzled as to just what a prism does when placed before an eye. They refer to every available volume and are often confused between ductions and phorias, finally dropping the subject as an unsolvable problem. In view of this fact, it is suggested that the refractionist should read the present volume with the actual instrument before him. Before proceeding, one should first understand the effect of a prism and what it accomplishes. To determine this, close one eye, looking at some small, fixed object; at the same time, hold a ten degree prism base in before the open eye, noting displacement of the object. This will clearly show that the eye behind the prism turns toward the prism apex. To carry the experiment further, the following test may be employed on a patient. Covering one eye, direct his attention to a fixed object, placing the ten degree prism before the eye, but far enough away to see the patient’s eye behind it. As the prism is brought in to the line of vision, it will be seen that the eye turns towards the apex of the prism. When the prism is removed, the eye returns to its normal position. Similar experiments enable the refractionist to make the most practical use of treating phorias and ductions, as well as to comprehend all other technical work. [Illustration: Fig. 13—An important part of the equipment for muscular work.] THE PHOROMETER