The peculiar appearances known as spectres in optics are certain illusions of vision in which an object is apparently presented to the view which does not really exist. In such cases either the brain, the retina, or the optic nerve are unnaturally excited, and made sensitive to an appearance that, physically speaking, does not exist. There is such a close connexion between the senses and the mind, that we continually, and without knowing it, transfer to the physical world that which belongs to the domain of thought. A picture which has struck us during the day will reappear to us at night during sleep, with every detail perfect, or possibly under a form modified by the capricious wanderings of our thoughts. A sudden fright may sometimes be the cause of optical illusions which will pursue us unceasingly. Fear, despair, passion, ambition, and other violent mental phases, are capable of evoking images closely connected with the state of our brain, appearances that we often take for realities, and whose truths we have to test by our faculty of reasoning, before we can set them down as positive illusions. “In the most insignificant phenomena,” says Sir David Brewster, “we find that the retina is so powerfully influenced by exterior impressions as to retain the images of visible objects for a long time after they have passed out of sight; besides, this portion of the eye is so strongly influenced by local impressions of which we know neither the nature nor the origin, that we see the shapeless forms of coloured light moving about in the dark. In fact we have, in the cases of Newton and many others, examples of the ease with which the imagination revivifies the images of luminous objects for months or even years, after these impressions took place. After the occurrence of such phenomena, the mind can readily comprehend how thin is the division that separates reality from those spectral illusions which during a particular state of health have afflicted the most intelligent men, not merely those belonging to the community at large, but also the most learned philosophers.” Spectres may properly be divided into two classes, those which may be termed subjective, which result from some unnatural action of our minds or bodies, and which properly belong to the science of physiology, and those which may be called objective, which are caused by some peculiar illusion acting on us from without. We shall pass lightly over the first, illustrating them by a single example, while we shall pay more serious attention to those belonging to the second class.