In 1929 Berger (1) discovered that changes of electrical potential in the human brain could be detected through the unopened skull. Since that time the study of electro-encephalography has occupied the attention of many workers, and the literature is already too extensive for adequate review in this place. A brief description of the technique for obtaining an electro-encephalogram, or “EEG”, and a summary of its normal and pathological characters may be found in a communication by one of us (2) on the relation between the EEG and the presence of intracranial neoplasms. The cortex in the region of a tumour was found to produce abnormally slow potential waves, which were provisionally called “delta” (δ) waves to distinguish them from the normal “alpha” (α) waves which are the original “Berger rhythm”. In the same paper a case was reported in which a focus of δ waves was found in the left parieto-occipital region associated with an area of degenerating cortex. The history in this case was of occasional minor attacks and one major fit and an indefinite severe illness in infancy, the only sign being a right homonymous hemianopia in accordance with the left-sided focus. Ether and nitrous oxide anæsthesia are also accompanied by the production of slow waves, but in this condition there is no fixed focus, the whole cortex being engaged in abnormal electrical activity. Since the publication of the above-mentioned results, a case of cerebral abscess has been examined, and the EEG was found to indicate a δ focus similar in character to those which have been found in cases of new growth.