Subject of spectral illusions--Recent and interesting case of Mrs. A.--Her first illusion affecting the ear--Spectral apparition of her husband--Spectral apparition of a cat--Apparition of a near and living relation in grave-clothes, seen in a looking-glass--Other illusions, affecting the ear--Spectre of a deceased friend sitting in an easy-chair--Spectre of a coach-and-four filled with skeletons--Accuracy and value of the preceding cases--State of health under which they arose--Spectral apparitions are pictures on the retina--The ideas of memory and imagination are also pictures on the retina--General views of the subject--Approximate explanation of spectral apparitions. The preceding account of the different sources of illusion to which the eye is subject is not only useful as indicating the probable cause of any individual deception, but it has a special importance in preparing the mind for understanding those more vivid and permanent spectral illusions to which some individuals have been either occasionally or habitually subject. In these lesser phenomena, we find the retina so powerfully influenced by external impressions, as to retain the view of visible objects long after they are withdrawn: we observe it to be so excited by local pressures of which we sometimes know neither the nature nor the origin, as to see in total darkness moving and shapeless masses of coloured light; and we find, as in the case of Sir Isaac Newton, and others, that the imagination has the power of reviving the impressions of highly luminous objects, months and even years after they were first made. From such phenomena, the mind feels it to be no violent transition to pass to those spectral illusions which, in particular states of health, have haunted the most intelligent individuals, not only in the broad light of day, but in the very heart of the social circle. This curious subject has been so ably and fully treated in your Letters on Demonology, that it would be presumptuous in me to resume any part of it on which you have even touched; but as it forms a necessary branch of a Treatise on Natural Magic, and as one of the most remarkable cases on record has come within my own knowledge, I shall make no apology for giving a full account of the different spectral appearances which it embraces, and of adding the results of a series of observations and experiments on which I have been long occupied, with the view of throwing some light on this remarkable class of phenomena.