Page:The wonders of optics (1869).djvu/272



close our account of the wonders of optics by a description of the ghost illusion, which has been exhibited with such great success by M. Robin, the well-known French conjurer, Mr. Pepper, the enterprising manager of the Royal Polytechnic Institution, and several others. Before doing so, however, we will say a few words on those unpleasant visitations known as spectres, to which some people are liable, either through an over-worked brain or some organic disease.

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 men