In muscular asthenopia, however, in addition to becoming easily tired, the patient often complains that letters seem to jump or run together or he may contend that he sees double for an instant; or again that he can “feel his eyes turn” involuntarily in their sockets. These pains or conditions are sometimes present only during actual _use_ of the eyes. At other times they persist for hours. In some cases, after days or weeks of overstimulation, an explosion in migraine form occurs at irregular intervals. This condition often lasts a day or two. _Reflex Symptoms._ In the third and last case, there are other reflex symptoms—such as dizziness, nausea, fainting, indigestion, insomnia and pains in other portions of the body—sometimes stimulating organic diseases. The possibility of heterophoria as a factor in chorea, migraine, neurasthenia and other diseases which may be primarily due to unstable nerves, equilibrium is not to be forgotten. It is a notable fact that when the fusion compensation fails so completely that one image is entirely suppressed, or the diplopia is so great as to be overlooked, the symptoms often cease entirely. TREATMENT The treatment of heterophoria depends on a careful study of each individual case, but it cannot be too strongly emphasized that in the great majority of cases the subjective symptoms disappear after a full correction of the refraction is made. In many cases, if the visual acuity in each eye be made normal, the fusion impulse alone will be sufficient to restore compensation. Many cases of esophoria result from overstimulation of the centers for convergence and accommodation, made necessary by hyperopia and astigmatism, entirely disappearing when glasses abolish the need of accommodation. Cases of exophoria are sometimes due to the abnormal relaxation of accommodation and convergence which secures the best distant vision in myopia. Likewise the correction of myopia, by increasing the far point, may diminish the amount of convergence necessary for near vision. Prisms for constant use are often prescribed, so placed as to help the weak muscles and counteract the strong. For instance, in esophoria we find the prism which, base in, will produce orthophoria for distance and prescribe a quarter of it, base in, before each eye. While this is very successful in some cases, the tendency in others is for the externus to increase slightly from constant exercise in overcoming the prism, while the internus decreases in proportion to the amount of work of which it is relieved. Prisms for permanent use are very beneficial in vertical deviations, since when the images are brought on the same level they require much less effort to secure fusion; and when prescribed base up or down, the effect secured is commonly an unchanging one.