The various theories about the cause of hallucinations have been largely influenced by the material which the writers studied. Thus, Mayer-Gross formed his theory on observation of the fantom limb, Schroeder on observation of delirious states and Vogt on experience with electrical stimulation of the cerebral cortex. It lies in the nature of the subject that its study is based mainly on verbal descriptions, given by patients with more or less impaired ability to describe their experiences. It was realization of this drawback which made so attractive the study of hallucinations experimentally produced in normal subjects by drugs such as mescaline. Work by Mayer-Gross, Stein, Zucker and others has proved the fruitfulness of this study of "experimental psychosis." Mescaline hallucinations are predominantly, though not exclusively, visual, so that a description of them by means of drawings and pictures could be expected to be somewhat more impressive, and perhaps more realistic,