The execution of one or other of these forms of measurement is in every case to be recommended, and if their exactness is not as perfect as can be desired, still, on the other hand it should be remembered, that for surgical treatment, an exact measurement of the deviation does not possess the importance sometimes assigned to it, as in most cases the squint angle shows considerable variations.
In a large number of cases these variations are so great, that a correct position of the eyes alternates with a more or less considerable squint, which as the case may be, appears seldom or often, sometimes only under certain conditions, and sometimes quite unexpectedly (periodic squint). In some cases stationary or permanent squint begins with the periodic form, however, one must not conclude that periodic squint is invariably the precursor of the permanent form. In by far the greater number of cases periodic squint continues unchanged without ever becoming permanent.
The transition from squint to the normal condition is formed by those cases, in which the proper position of the eyes is maintained by a desire for binocular single vision, while the elastic tensions of the muscles are such, that squinting sets in as soon as binocular single vision is rendered impossible (latent squint).
The squint is generally one sided (monolateral), for the eyes in this case are usually of unequal value, and the best is always preferred for use. The eye which has the acuter vision is always made use of when something has to be carefully observed. But when the acuteness of vision is equal, and one eye is emmetropic and the other hypermetropic, or if both are hypermetropic but in varying degree, the most hypermetropic eye is always the squinting one; for with a greater power of accommodation it does not accomplish more than the emmetropic or less hypermetropic one with slighter expenditure of strength. Why should a man strain his accommodation when no advantage is thereby gained?
In most cases the squinting eye has also an available power of vision and is on that account used for fixing objects which lie in the direction of its visual axis; it can also be made to fix objects in front, this occurs as soon as the other eye is covered; it remains as the fixing eye till the next blinking of the lids, or movement to another object for fixation, or till both eyes are closed for a short time, when it returns to its former deviation.
A true alternating strabismus, i. e. alternate use of first one eye and then the other to fix objects straight ahead, only occurs when both eyes are of equal value as regards weakness and acuteness of vision, or when one is more conveniently used for near, and the other for distant vision. In these circumstances one eye is always short-sighted and is used for near objects, while the other is emmetropic (or in less degree near-sighted or long-sighted) and is preferred for distant things. The reason for the alternation lies in the necessity for the act of vision itself; it begins regularly whenever distant and near objects are alternately fixed. Alternating squint is usually divergent, with short sight on one side, still convergent strabismus may occur under these conditions.
CONVERGENT SQUINT
To Donders belongs the merit of having pointed out the presence of hypermetropia in about two thirds of all cases of convergent strabismus. The fact is undeniable, the theories built upon it are doubtful. Donders declares no other conclusion to be possible, than this, that the hypermetropia is the cause of the squint. "To see clearly, the hypermetrope must accommodate vigorously for each distance. In looking even at distant objects he must overcome his hypermetropia by exerting his accommodation, and in proportion as the object approaches him, he must add to it as much accommodation as the normal emmetropic eye would use. The inspection of near objects requires then a special amount of exertion. There exists, however, a certain connection between accommodation and convergence of the visual lines. The stronger one converges the more one has to put into action the accommodation. A certain tendency to convergence cannot then be absent during any effort of the faculty of accommodation."