As the revolving card is virtually transparent, so that bodies beyond it can be seen through it, the power of the illusion might be greatly extended by introducing into the picture other figures, either animate or inanimate. The setting sun, for example, might be introduced into a landscape; part of the flame of a fire might be seen to issue from the crater of a volcano, and cattle grazing in a field might make part of the revolutionary landscape. For such purposes, however, the form of the instrument would require to be completely changed, and the rotation should be effected round a standing axis by wheels and pinions, and a screen placed in front of the revolving plane with open compartments or apertures, through which the principal figures would appear. Had the principle of this instrument been known to the ancients, it would doubtless have formed a powerful engine of delusion in their temples, and might have been more effective than the optical means which they seem to have employed for producing the apparitions of their gods. In certain diseased conditions of the eye, effects of a very remarkable kind are produced. The faculty of seeing objects double is too common to be noticed as remarkable; and though it may take place with only one eye, yet, as it generally arises from a transient inability to direct the axis of both eyes to the same point, it excites little notice. That state of the eye, however, in which we lose sight of half of every object at which we look, is more alarming and more likely to be ascribed to the disappearance of part of the object than to a defect of sight. Dr. Wollaston, who experienced this defect twice, informs us that, after taking violent exercise, he “suddenly found that he could see but half of a man whom he met, and that on attempting to read the name of JOHNSON over a door, he saw only SON, the commencement of the name being wholly obliterated from his view.” In this instance, the part of the object which disappeared was towards his left; but on a second occurrence of the same affection, the part which disappeared was towards his right. There are many occasions on which this defect of the eye might alarm the person who witnessed it for the first time. At certain distances from the eye one of two persons would necessarily disappear; and by a slight change of position either in the observer or the person observed, the person that vanished would reappear, while the other would disappear in his turn. The circumstances under which these evanescences would take place could not be supposed to occur to an ordinary observer, even if he should be aware that the cause had its origin in himself. When a phenomenon so strange is seen by a person in perfect health, as it generally is, and who has never had occasion to distrust the testimony of his senses, he can scarcely refer it to any other cause than a supernatural one.