Gases. Lenses and mirrors. Disease of the eye. In the first class there is no real or palpable object, or, if there be, it is not what it appears; the illusion is but the reality of romance, depending altogether on excited or disordered conditions of the mind: the source, therefore, either of bright or gloomy phantoms, as the mood may be. On this scroll I have recorded those _moods of mind_, which, excited by memory or association, or influenced by such casualties as solitude, moonlight darkness, or localities of interest, or the poring over tales of horror at midnight, may be considered the _predisposing causes_ of illusion. Such are:— Temperament Credulity, Enthusiasm, Superstition, Timidity, Imagination, Poetic frenzy. Excitement Sympathy, Exalted joy, Deep grief, Love, Hatred, Protracted anxiety, Delirium of fever, Delirium of alcohol, Delirium of narcotics, Exhaustion, Disease of the brain. The second class, which are spectres or ghosts of the eye, may be scientifically explained by the laws which govern the _material world_. These are the only substantial ghosts which I can grant to my friend. The objects themselves exist, and are exactly as they appear. The philosopher regards them as interesting exceptions to general rules, from _peculiar_ combinations of _natural_ causes. The unlearned will term them preternatural _phenomena_, simply because they are of uncommon occurrence. But which among the works of divine creation is not a phenomenon? We may think we know a law of nature, but can we analyse it? Novelty and magnitude astonish, but that which is familiar excites not our surprise. We gaze with delight on the progress of an eclipse; we watch with wonder the eccentric course of the comet; but we look on the sun in its meridian glory with a cold and apathetic indifference. Yet do they all alike display Divine Omnipotence, and the expansion of a vegetable germ, the bursting of a flower, is as great a miracle as the overwhelming of a deluge, the annihilation of a mighty world. To discriminate between these classes is not difficult: we may prove their nature by simple experiment. _Optical illusions_ will be _doubled_ by a straining or altering of the axes of the eyes; and, by turning round, as they are removed from the axis of vision, they will disappear. So, indeed, will those of the second class, which are _real_ objects converted into phantoms by mental excitement or disorder.