Whether squint originates in the permanent or periodic form depends chiefly on whether the movement of convergence is retained or lost. There are cases of considerable divergent squint, in which the near point of the convergence is scarcely removed, while on the other hand, the physiological innervation for convergence may be lost, without absolute divergence ever being brought about. In a number of emmetropic or slightly myopic cases with absolute preponderance of the externi, the physiological connection between accommodation and convergence is maintained in a relaxed way; thus, for example, it is impossible to converge voluntarily to a large object, as, for instance, a pencil held in the vertical line, while accurate convergence immediately follows on reading at the same distance; in other cases accommodation can be exerted to the near point, without inducing the slightest impulse to convergence. This circumstance is worthy of consideration for the prognosis of the operation. A mere relaxing of the tie between accommodation and convergence may be strengthened by practice, but if the impulse to innervation is completely lost, it will scarcely be possible to restore it again; as after complete laying aside of absolute divergence the relative form still continues to exist. Those cases deserve special consideration in which emmetropia is present in one eye, in the other myopia. Slight degrees of one-sided myopia reconcile themselves with the continuance of a normal binocular act of vision. If the far point of the myopic eye lies at an inconvenient proximity even for reading, then, as a rule, the emmetropic eye is used for near as well as distant objects; if, on the contrary, the degree of myopia answers to a range of vision convenient for working, and visual acuteness is normal, then the temptation to use the emmetropic eye only for distance and the myopic one only for near objects is so overpowering, and the advantages on the other hand which would be offered by clinging to binocular vision so slight, that a convenient monocular vision is generally preferred. Even for objects which lie nearer the eye than the far point of the myopic, and at the same time farther than the near point of the emmetropic eye, for which, therefore, both eyes could secure clear retinal images, binocular vision is not used. In cases in which the patient can read with proper binocular fixation, if one covers all but one line and then makes with prisms double images standing one above another, it is the myopic eye alone which almost invariably shows a clear retinal image.