The present volume, dealing with the instrument’s distinctive operative features, has been prepared not only for Ski-optometer users, but also for those interested in the simplification of refraction and muscular imbalance. The author is indebted for invaluable counsel, to Louis J. Ameno, M.D., New York. E. LeRoy Ryer, O.D., New York. Jos. D. Heitger, M.D., Louisville, Ky. W. B. Needles, N.D., Kansas City, Mo. INTRODUCTORY While in a measure the conventional trial-case still serves its purpose, so much of the refractionist’s time is consumed through the mechanical process of individually transferring the trial-case spheres and cylinder lenses, that far too little thought is given to _muscular imbalance_, notwithstanding its _importance_ in all refraction cases. Dr. Samuel Theibold, of Johns Hopkins University, in a recent address before the American Medical Association, stated that the average refractionist was inclined to devote an excess of time to general refraction, completely overlooking the important test and correction of muscular imbalance. If the latter is to be at all considered, general refraction must be simplified—without impairing its accuracy—a result that is greatly facilitated through the use of the Ski-optometer. One must admit that tediously selecting the required trial-case lens—whether sphere, cylinder or prism—watching the stamped number on the handle—continual wiping and inserting each individual lens in a trial-frame is a time-consuming practise. This is readily overcome, however, through the employment of the Ski-optometer. In a word, the Ski-optometer is practically an automatic trial-case, bearing the same relation to the refracting room as the accepted labor and time-saving devices of the day bear to the commercial world. The present volume has accordingly been published, not alone in the interest of those possessing a Ski-optometer, but also for those interested in attaining _the highest point of efficiency_ in the work of refraction and muscular imbalance. [Illustration: _Ski-optometer Lens Battery (almost actual size) showing how sphere and cylinder lenses are procured._ _After obtaining FINAL results, your prescription is automatically registered, ALL READY for you to transcribe._ Fig. 1—The three time-saving moves necessary in the operation of the Ski-optometer.] CHAPTER I SKI-OPTOMETER CONSTRUCTION A far better understanding of the instrument will be secured if the refractionist possessing a Ski-optometer will place it before him, working out each operation and experiment step by step in its proper routine. The three moves as outlined in Fig. 1 should first be thoughtfully studied and the method of obtaining the spheres and cylinders carefully observed. [Illustration: Fig. 2—To Obtain Plano.] 1—Set spherical indicator at “000” as illustrated above. 2—Set cylinder indicator to “0”. 3—Set pointer of supplementary disk at “open”.