To the illusions to which, like the preceding, we are all subject, may be added those resulting from some abnormal conformation, or some disease of the eye, in those who labour under them. An example of this occurs in the case of double or triple vision, many remarkable instances of which are mentioned by Müller, the celebrated physiologist.
Although, as before explained, the image of an object is depicted at the same time on both our eyes, still we only see one impression, in consequence of the two images being carried to the brain from corresponding portions of the retina. If this relation be disturbed by any cause, or if the eyes are not converged exactly upon the same point, a double image is the result. The first of these facts may be proved by looking at the moon, for instance, with the left eye shut; on suddenly opening it, two images will be seen for an instant. The second is instantly proved by pushing either of the eyes aside with the finger, when looking at any object.
It is necessary, however, to distinguish between these effects and true double vision, as well as a certain defect which exists in the eyes of many people, consisting in the apparent multiplication of distant objects by the same eye. In these cases, there is a superposition of images upon the retina, each having its proper bounds. With the majority of individuals afflicted in this way, it only happens when they look at a very distant object, the moon or stars for instance. There are many, however, who suffer from it in the case of everything they look at, whether far or near. Stephenson, who was affected with it, made it the subject of many interesting experiments. When he looked at a clear mark on a white ground, and gradually walked away from it, not only did the image become indistinct, but it seemed to unfold itself into several, independently of many others much
