If we turn to the mechanism of hallucinations, we shall find that--like dreams--some are apparently originated by the condition of the bodily organs; others again appear to be mere automatic reverberations of recent sensation; whilst yet others cannot be referred to any immediate external stimulus, and suggest the "spontaneous" activity of the higher cerebral centres. With the rudimentary hallucinations--singing in the ears, sparks and flashes of light, etc.--which are caused by transient conditions of the external organs of sense, we are probably all familiar. But experience shows that a small nucleus of actual sensation may enter into more fully developed hallucinations. Thus, to take the simplest case, it is known that "sparks" may develop into "Faces in the Dark," which are themselves on the border-line between mind's-eye pictures and hallucinations. (See _St. James's Gazette_, "Faces in the Dark," Feb. 10, 1882, and _Proc. S.P.R._, vol. iii. p. 171.) And in another recorded case (_Proc. S.P.R._, vol. i. pp. 102, 103) an artist was accustomed to see constantly at his studio the figure of a man, under circumstances which strongly suggest that a _point de repère_ was furnished by those floating motes in the eyeballs which are liable momentarily to cloud the vision when the position is abruptly changed after a period of immobility. And we find cases where the constructive impulse has so amplified and misinterpreted the data of normal sensation that we hardly know whether to class the result as hallucination or illusion. Thus, in a case given in _Phantasms_ (vol. ii. p. 28), a young girl sees the face of a friend growing out of a yellow pansy; and an account of a similar incident has recently been furnished to me by Mr. H. Smith, of the Central Telegraph Office. The reference in the first line of the following narrative is to a rumour of the house being haunted, the remembrance of which possibly gave a definite form to the apparition:-- HALLUCINATIONS CLASSIFIED ACCORDING TO THE SENSE AFFECTED AND ACCORDING TO THE KIND OF PERCEPT.