Physical optics is an exact science. When used for refraction of human eyes, however, it becomes an art, because it deals with two uncertain variables, the examiner and the person examined. It will reflect the reactions of each. No matter how careful and painstaking the examiner and how cooperative the patient, there are many patients who still complain of eye strain. This is due to the fact that some things must be estimated, according to the response of the patient and the judgment of the physician. Many of these unsatisfactory cases result from incorrect estimation of the interpupillary distances for far and near work. The methods of estimating the interpupillary distance (P. D.) are too well known to require much discussion. The most frequently used for distance is the method of squinting first with the right eye and then with the left over the edge of a ruler held in