Page:The wonders of optics (1869).djvu/74

 That such hallucinations have no real existence as far as the eye goes, is proved by the fact of many people who have lost their sight, being subject to them. It is hardly to be wondered at that those who by accident have been deprived of their sight, should wish so ardently to see once more the persons and sights they have taken pleasure in, that they should at last create for themselves illusions of this character. The same thing has frequently occurred with those whose sight is more or less weak. An old man of eighty, who was purblind, never sat down to a table during the last years of his life, without seeing around him a number of his friends who had long been dead, dressed in the costume of fifty years before. This old man had but one eye, which was extremely weak, and wore a pair of green preservers, in the glass of which he continually saw his own face reflected.

Doctor Dewar, of Stirling, mentioned to Abercromby a very remarkable instance of this species of hallucination. The patient, who was quite blind, never walked in the street without seeing a little old woman hobbling on before him and leaning on a stick. This apparition always disappeared when he entered his house.

Similar illusions frequently happen to every one, even the most healthy amongst us, but a little consideration soon puts them to flight. It would be useless to mention the numberless cases in which a square tower has appeared round, or where the landscape has suddenly seemed to recede from the sight. Such illusions as these have been long well known, and appreciated at their proper value; but there are others whose true cause has remained a mystery, until explained by the progress of science, such as the Spectre of the Brocken, the Fata Morgana, and the mirage.

Analogous appearances have been seen in Westmoreland and other mountainous districts, the inhabitants