But although both eyes see at the same time, yet the close relation which in binocular fixation develops between the centres of the retinæ does not occur in squint; firstly, because the retinal area in the squinting eye which corresponds to the fixation point is too excentric, and secondly, because the angle of the squint often changes. In binocular fixation, the fixation point of one retina answers to the corresponding point of the other; in squint, on account of the varying size of the squint angle, if a like relation develops between the eyes, the fixation point of one retina must correspond to a larger area of the other. Possibly this explains a fact that is often to be observed. In those cases of squint where diplopia can easily be caused by covering one eye with a red glass and the other with a vertically deviating prism, the double images disappear on rotation of the prism round the axis of vision, as soon as the angle of the prism reaches an angle of about 45°. The occurrence of double images shows that there exists for the upper and lower parts of the retinæ a community of vision by no means coinciding with the identity principle. The disappearance of the diplopia can be explained by the fact that the variations of the squint angle take place chiefly in the horizontal direction. Therefore the area in the squinting eye that corresponds to the fovea centralis of the fixing eye must be more extensive in the horizontal than in the vertical direction. Alfred Graefe has designated this phenomenon as "regional exclusion." Whilst then a sort of community of vision exists for the upper and lower parts of the retinæ, the sensations of the retinal area lying in the horizontal plane of the macula lutea of the squinting eye must be suppressed. The physiological occurrence of a suppression of the retinal images, as far as we are able to investigate it, always refers to the whole retina; however, the possibility of a "regional exclusion" should not be excluded to begin with; but in the inductive sciences it is for us to ask first, whether an incident really happens, and not whether it is possible. The fact from which Alfred Graefe draws his inference is not, as we have just seen, to be explained in any other way, and the ophthalmoscopic test described on p. 65 proves that also in these cases of "regional exclusion" both eyes are used for vision.
In many cases of periodic squint the condition of binocular vision is very interesting. Binocular fusion may be quite absent even in normal position of the eyes; on the other hand the non-occurrence of diplopia in squint does not prevent the occurrence of perfect binocular fusion with a normal position. In periodic outward squint I have sometimes seen binocular fixation without the existence of binocular fusion; the excluded eye deviates outwards, but as soon as it is free it puts itself into fixation, whilst neither with prisms nor stereoscope can anything other than alternating vision be proved, i. e. neither binocular diplopia nor fusion.
If squint arises when the habit of binocular single vision has become confirmed, diplopia is always present, at least at first; even children of six to seven years old make this statement uninvited, but they soon get accustomed to the new relations, and after a short time it is impossible to make them see double images (see Case 42). Habits cling more closely in adults, therefore that form of convergent squint in particular, which usually develops quickly in myopia of average degree, causes annoying diplopia to last for a longer time. For just when these patients want to employ binocular vision in order to estimate distance correctly, diplopia occurs to hinder and confuse them.
It is otherwise with the relative divergence which is developed in consequence of myopia. At first diplopia is present here for a short time; in this case circumstances are specially favorable to a temporary suppression of the deviating eye; the fixing eye receives large distinct images to which the attention is directed. Meanwhile the relatively divergent eye is usually turned to other more distant objects that furnish indistinct retinal images, from which the attention is easily diverted. The habit of suppression may become so dominant that binocular fixation continues to exist for distant objects and the presence of binocular fusion is easily traceable, while for near objects, which are monocularly fixed with relative divergence, it is impossible to render the patient conscious of the images of the deviating eye.
Considerable squint is by no means necessary for the cessation of normal binocular single vision; slight, frequently recurring deviations are quite sufficient, as in those cases where want of control renders physiological innervation for convergence more difficult. Double images are present here, although not in a troublesome way, as is usual in relative divergence, but binocular single vision does not exist even for distance. The reason for this does not lie in the impossibility of fixing the same object simultaneously with both eyes, for the objectively proved deviation may be extremely slight. A union cannot be obtained even by prisms. If crossed double images are present close together, a prism of a few degrees base inwards suffices to make them homonymous. The habit of binocular single vision is lost, in consequence of that disturbance to the innervation of the interni which is designated as insufficiency of the same.
The stereoscope, as well as the prism, is useful for testing binocular single vision, especially when it is suitably modified for the purpose. The prismatic glasses usually attached to stereoscopes are here quite superfluous. The advantage of the prismatic deviation consists solely in the fact that the centres of the images fixed for the macula lutea on each side can be removed farther from one another than the distance apart of the eyes amounts to, so that a greater extension of the visual area is rendered possible. Ordinary stereoscopic pictures are quite useless for testing binocular vision; it is a question here of employing diagrams, which contain on the one hand very prominent identical figures stimulating binocular fusion but which, on the other hand, offer for each eye special attractions not present in the visual field of the other. Further, it is desirable to regulate the stereoscope so that the glasses are not firmly inserted, but that glasses from the trial case may be applied according to the condition of refraction of the patient and the distance of the stereoscopic images.
The stereoscope is generally used with the greatest advantage in those cases where there is no conspicuous deviation, and by testing binocular vision conclusions may be drawn as to whether normal binocular fusion exists or has disappeared in consequence of the squint.
It is desirable to use both methods of investigation, that with the stereoscope as well as prisms, as each test has its own value. One who at once combines the stereoscopic fields of vision certainly has binocular single vision; in other cases this is only so far lost that the stereoscopic combination does not take place at once but only after some trouble. Care must be taken, especially when one eye has defective vision, that the corresponding visual field contains objects sufficiently large and easily recognisable, as very small objects which do not correspond to the lowered visual acuity are easily overlooked. It sometimes happens that both fields are seen at the same time, but that there is no fusion; finally it happens frequently that there is complete suppression of one visual field. In testing with prisms it may appear doubtful as to whether binocular fusion or suppression of one eye exists; however, the stereoscope at once gives us certain information. It must not be forgotten that the altered relations between the eyes, which are always possible in squint, also appear at the same time; he who sees double with prisms, may yet be able completely to suppress the stereoscopic visual field of one eye. Binocular fusion, suppression of the squinting eye and simultaneous vision with both eyes without binocular fusion can alternate in the same individual. Von Kries has come to the same conclusion, and if our colleague is unable to explain all the phenomena of binocular vision that he could observe in his own case, we need not be astonished if we sometimes hear from our patients statements that appear incomprehensible and unphysiological.
At any rate it is evident that the absence of diplopia in squint can easily be understood, without adopting the arbitrary idea of a constant, habitual suppression of the image of the squinting eye.