According to the law of associated movements, when an eye squinting upwards is put into fixation, the other eye should make a movement downwards, as normally both eyes move together up and down, yet this is not always the case. For example, when an upward deviation is present in convergent squint, it not uncommonly follows that the secondary deviation of the eye which usually fixes is also inwards and upwards; only exceptionally in cases of deviation in height of the squinting eye does the sympathetic movement take place without change of height. Sometimes with deviation of height, I found combined a distinct rotation of the eye, generally thus, that together with the movement upwards was combined a rotation of the vertical meridian outwards and _vice versâ_; in fixing the eye a rolling inwards was combined with the movement downwards. The other eye then usually showed a similar rotation (thus the meridian of both eyes rotated simultaneously to the right or left), but the deviation in height was not always the same. The law of equal innervation requires in alternate fixation, first with one eye, then with the other, that the same degree of deviation be transferred to the non-fixing eye. When exceptions appear, and the deviation in the two eyes is unequal, it is (provided the inequality has not been caused by attempted operation, or is the result of paralysis), usually to be explained by the fact, that an accommodative movement takes place when we are expecting an associated one. For example, if there is convergent squint and hypermetropia in both eyes, but more hypermetropia in one than the other, in alternate fixation it will be found that the least hypermetropic eye always undergoes the greatest deviation, because in fixation with the more hypermetropic eye a stronger effort of accommodation unites itself with a corresponding innervation of the internal rectus, which is transferred equally to the other and non-fixing eye. Thus it happens frequently in divergent strabismus, when one eye is myopic, the other emmetropic. If the latter fixes an object stationed near the "far point" of the myopic eye, the internal recti and the accommodation act simultaneously; on the other hand if the myopic eye fixes, it wants no accommodation and the emmetropic eye sinks into divergence. With regard to the immutability of the squint; it must not be understood that the squint angle always remains the same with the same individual; in most cases the amount of deviation varies, the squint is now less, now greater; it is desirable however, to know the bounds within which it fluctuates. To determine the degree of the squint one can either ascertain the angle of the squint, or use v. Graefe's so-called linear measure of deviation.