Physiology of perception and of dreaming--Definition of the act of dreaming--Revery--Production of illusions and hallucinations by drugs and by disease--Effects of hasheesh--Effects of acute disease--Association of ideas--Memory of past sensations--Dreams produced by excitement of the different organs of sense--Persistence of dream-impressions after waking--Experience of M. Baillarger--Of Professor Jessen--Belief of savages in the reality of dreams--Sensory dreams--Intellectual dreams-- Repetition of dreams--Incoherence of dreams--Cause of the superior vividness of certain dreams--Duration of dreams-- Dreams excited by morbid states of the body--Prophetic dreams--Their causes--Clairvoyant dreams--Hallucinatory dreams--Sir Edmund Hornby’s experience--Hallucinations-- Case related by Dr. E. H. Clarke--Revelation through dreams--Revival of memory in dreams. 116 CHAPTER VI. SOMNAMBULISM. Causes of somnambulism--Physiology of somnambulism-- Varieties of the disorder--Maury’s classification-- Classification of Ball and Chambard--Diagrammatic representation of their classification--Somnambulic lethargy--Illustrative cases--Somnambulic dreams--Night terrors--Somnolentia or sleep-drunkenness--Sleep-walking-- Illustrative cases--Condition of the special senses in somnambulism--Relation of memory to the somnambulic paroxysm--Illustrative cases--Occasional recollection of incidents connected with the somnambulic dream-- Resemblances between the somnambulic state and the condition of post-epileptic mania--Somnambulic visions--J. P. Frank’s case--Mesnet’s case--Somnambulic life--Its likeness with the double-consciousness of certain forms of epilepsy--Illustrative cases--General theory of somnambulism. 166 CHAPTER VII. ARTIFICIAL SOMNAMBULISM OR HYPNOTISM. Antiquity of the phenomena of hypnotism--Modern observations--Physical conditions favorable to the phenomena--Methods of inducing the hypnotic state--Duration of hypnotic sleep--Rudimentary states of hypnotism-- Investigations of the Society for Psychical Research-- Mind-reading--Physiological explanation of the process-- Charcot’s observations on artificial somnambulism-- Cataleptic variety of the hypnotic state--Lethargic variety--Somnambulic variety--Hypnotic clairvoyance-- Exalted sensibility of the brain in hypnotic states-- Susceptibility to suggestions from without--Phenomena of so-called spiritualism--Table-rapping--Planchette-- Therapeutical employment of hypnotism--Metaphysical healing. 212 CHAPTER I. THE NATURE AND CAUSE OF SLEEP. Worn out, friend, is every theory, But green the golden tree of life. --GOETHE.