An apparently healthy young lady suddenly fell into an abnormally long and deep sleep--it is said without prodromal symptoms. On awaking she had forgotten the words for and the knowledge of the simplest things. She had again to learn to read, write, and count; her progress was rapid in this re-learning. After a second attack she again woke in her normal state, but without recollection of the period when she had forgotten things. These states alternated for more than four years, during which consciousness showed continuity within the two states, but was separated by an amnesia from the consciousness of the normal state. These selected cases of various forms of changes of consciousness all throw a certain light upon our case. Naef's case presents two hysteriform eclipses of memory, one of which is marked by the appearance of delusions, and the other by its long duration, contraction of the field of consciousness, and desire to wander. The peculiar associated impulses are specially clear in the cases of Proust and Mesnet. In our case the impulsive tearing up of the flowers, the digging up of the graves, form a parallel. The continuity of consciousness which the patient presents in the individual attacks recalls the behaviour of the consciousness in MacNish's case; hence our case may be regarded as a transient phenomenon of alternating consciousness. The dreamlike hallucinatory content of the limited consciousness in our case does not, however, justify an unqualified assignment to this group of _double consciousness_. The hallucinations in the second state show a certain creativeness which seems to be conditioned by the auto-suggestibility of this state. In Mesnet's case we noticed the appearance of hallucinatory processes from simple stimulation of touch. The patient's subconsciousness employs simple perceptions for the automatic construction of complicated scenes which then take possession of the limited consciousness. A somewhat similar view must be taken about our patient's hallucinations; at least, the external conditions which gave rise to the appearance of the hallucinations seem to strengthen our supposition. The walk in the cemetery induces the vision of the skeletons; the meeting with the three boys arouses the hallucination of children buried alive whose voices the patient hears at night-time. She arrived at the cemetery in a somnambulic state, which on this occasion was specially intense in consequence of her having taken alcohol. She performed actions almost instinctively about which her subconsciousness nevertheless did receive certain impressions. (The part played here by alcohol must not be underestimated. We know from experience that it does not only act adversely upon these conditions, but, like every other narcotic, it gives rise to a certain increase of suggestibility.) The impressions received in somnambulism subconsciously form independent growths, and finally reach perception as hallucinations. Thus our