The site of a cerebral lesion can often be accurately determined by painstaking analysis of the history and clinical signs. Its location can be further defined by intracranial air studies, angiography, and electroencephalography, all of which have increased in value during the past few years as the result of technical advances. From these investigations it is possible in the great majority of cases to obtain a clear impression of the position of the tumour. The determination of the exact volume of neoplastic tissue, however, remains a very difficult matter. The intravenous injection of dyes such as brilliant green or fluorescein which appear to have the ability to combine selectively with certain tissues including tumours, has been evolved to meet this difficulty, but has not gained wide acceptance (Friedemann and Elkeles, 1932; Spatz, 1933; Broman, 1941; Friedemann, 1942; Sorsby, Dickson Wright, and Elkeles, 1942; Broman, 1944, 1945). This method, however, has recently come into vogue again following reports from the U.S.A. describing the successful localisation of brain tumours by means of a suitably arranged Geiger-Müller counter applied to the intact skull, two to four hours after injection of radioactive diiodofluorescein (Moore, 1947; Boyack, Moore, and Clausen, 1948; Moore, 1948; Moore, Peyton, French, and Walker, 1948; Moore, Peyton, Hunter, and French, 1948; Ashkenazy, 1949; and Loyal Davis, Martin, Ashkenazy, Le Roy George, and Fields, 1950).