We have before illustrated, by means of the pen and the ink-bottle, how one object is lost sight of when the other is attentively regarded, and a material picture or scene may be equally lost sight of, and a mental picture take its place in the eye, when we recall places or people we have seen or remembered. We have all heard numerous anecdotes of what is termed “absent-mindedness.” Some people are quite absorbed in study, and can see or hear no one in the room when deeply occupied. We may be satisfied then that “pictures of the mind and spectral illusions are equally impressions upon the retina, and only differ in the degree of vividness with which they are seen.” If we press our eyes the phosphorescence becomes apparent, and we can make a picture of the sun or a lamp visible for a long time to our closed eyes if we stare at either object for a few seconds, and shut our lids. So by increasing the sensibility of the retina we can obtain the image, and alter its colour by pressure on the eye. It is well known that poisons will affect sight, and belladonna applied to the eyes will so affect them as to render the sight _nil_, by enlargement of the “pupil.” If one is out of health there is practically a poisoning of the system, and when we have a “bilious headache” we see colours and stars because there is a pressure upon the blood-vessels of the eye. The effects of a disordered stomach, induced by drinking too much, are well known; objects are seen double, and most ghosts may be traced to a disordered state of health of mind or body, brought on by excitement or fatigue. We could relate a series of ghost stories,—some in our own experience, for we have seen a ghost equally with our neighbours,—but this is not the place for them. Although many apparently incontrovertible assertions are made, and many spectres have been produced to adorn a tale, we must put on record our own opinion, that every one could be traced to mental impression or bodily affection had we only the key to the life and circumstances of the ghost-seer. Many celebrated conjurers will convince us almost against our reason that our pocket-handkerchief is in the orange just cut up. They will bring live rabbits from our coat-pockets or vests, and pigeons from our opera-hats. These are equally illusions. We know what can be done with mirrors. We have seen ghosts at the Polytechnic, but we must put down all apparitions as the result of mental or bodily, even unconscious impressions upon the retina of the eye. There are numerous illusions, such as the Fata Morgana, the Spectre of the Brocken, etc., which are due to a peculiar state of the atmosphere, and to the unequal reflection and refraction of light. Those, and many other optical phenomena, will, with phenomena of heat and sound, be treated under METEOROLOGY, when we will consider the rainbow and the aurora, with many other atmospheric effects. CHAPTER XV ACOUSTICS.