The exact rules for the application of the methods of operation differ according to the nature of the case under consideration. If we contemplate first the largest group, that of the ordinary permanent convergent squint, the choice of the method is principally determined by the average degree of deviation, the condition of error, and the visual power, lastly by the mobility, particularly the outward movement of the eyes. If the visual power of both eyes is nearly the same, or if the squinting eye possesses such a visual acuteness that it can be used in fixation, it is advisable as a rule to arrange the relations of the muscles as equally as possible in both eyes--simple division of the internal recti is therefore, as a rule, to be performed in both eyes. If, on the other hand, the vision of the squinting eye is in a high degree defective, so that only the better one is used, it is generally advisable to confine the operation as far as possible to the squinting eye; in that case, tenotomy of the internal rectus and advancement of the external rectus is usually indicated in the squinting eye, and frequently suffices. Deviations which are so slight, that the careful division of both interni without loosening the conjunctiva at the front part of the muscle makes us fear an excessive result, are seldom the subject of operative treatment; if the deviation is slight but still a disfigurement, if it amounts to 3 to 4 mm., distribution between both eyes is suitable, because, when the squinting eye possesses requisite visual acuteness it is put into fixation more frequently after the operation than before. Under these circumstances, if the operation is confined to the squinting eye, and a sufficient result is thereby obtained, as soon as this eye is used for fixation a remarkable secondary deviation of the other eye occurs, which is not the case if the tensions of the muscles have been balanced by an operation on both sides. A deviation of 5 to 6 mm. may usually be balanced by means of simple double tenotomy if the conjunctiva is considerably loosened behind the caruncle; not unfrequently, however, we must be careful to strengthen the result by means of the after-treatment. Commonly, during the first twenty-four hours, the result appears to be quite satisfactory, whilst on the second or third day troublesome convergence again sets in. By practice of the outward movement we then usually obtain at once a perceptible improvement of the position. Both eyes are repeatedly turned as far as possible to the right and left, by which means is obtained on the one hand, exercise of the external recti, on the other, increase of the effect of the tenotomy of the internal recti. I order these exercises to be begun on the day after the operation.