We sometimes take advantage of this tendency when we prescribe for constant use weak prisms with the apex over the weak muscle, which gradually becomes strong from the exercise of overcoming it. This plan is effective only in patients who have a strong fusion impulse, and the prism selected must be weak enough to be easily overcome. We can accomplish the same effect by decentering the patient’s refraction lenses. For instance, a convex lens so placed that the visual line passes the reverse will be the case if the lens is concave. The amount of prismatic action depends on the strength of the lens and the amount of decentering, the rule being that every centimeter of displacement causes as many prism diopters as there are diopters in that meridian of the lens. Thus +1 sphere, or cylinder axis 90, decentered one centimeter outward, is equivalent to adding a one degree prism diopter lens, base out. DESTROPHORIA AND LAEVOPHORIA These are terms denoting a condition in which both eyes are capable of abnormal rotating toward the right or left, as the case may be. The movement in the opposite direction is most common. The patient can often rotate his eyes 60 degrees toward the right, and to perhaps only 40 degrees to the left. His position of rest is parallel with his visual lines, but to the _right_, in looking at objects directly in front, he is much more comfortable with his head turned slightly to the left. It is difficult to account for, except on the theory that definite movement of the eyes is rather to the right than to the left in most occupations. The position of the paper in writing at a desk tends toward dextrophoria; in reading, we move our eyes steadily from left to right and then begin a new line by a single brief movement to the left; the things that a man uses most—whether he be laborer or student—are kept within reach of the right hand, and in referring to them the eyes are constantly turned toward the right. However, when these conditions result from other imbalances, they must be treated more carefully. For instance, a patient whose right internus is paralysed or congenitally defective on looking to the left, has a cross diplopia which vanishes to the right; as a result, he soon assumes a habit of carrying his head in this position. Ordinarily, this will cause no discomfort; but if the left internus is so weak that it cannot follow the right externus to its position of greatest ease, the visual lines are evidently different and the case must be treated as an exophoria. If, on the other hand, the _left_ internus over-balances the right externus, the condition is an esophoria and must be treated as such. Similar reasoning applies to the conditions known as Anaphoria and Kataphoria, in which the visual lines are parallel to each other but directed up or down with regard to the horizontal plane of the body.