THE value of rontgenography in the diagnosis and localization of intracranial tumors is mainly restricted to the cases in which the neoplasm has affected the skull. In an analysis of the X-ray findings in one hundred cases of /brain tumor from Doctor Halsted's Clinic, Heuer and I 1 have shown that in only 6 per cent. of the cases did the tumor cast a shadow, and in these it was only the calcified areas that were differentiated by the X-rays from the normal cerebral tissues. In those instances (9 per cent. of our cases) in which a tumor (has encroached upon the sphenoid, ethmoid or frontal sinus, the invading portion casts a shadow in the r6ntgenogram. Such shadows are due to the displacement of the normally contained air by tissues which are less pervious to the X-ray. This group of shadows is of minor practical importance because the growth can be recognized by the destruction of the walls or bony septa of the sinuses. Since the X-rays penetrate normal brain tissues, blood, cerebrospinal fluid and non-calcified tumor tissue almost equally, any changes in the brain produced by altered proportions of these components will not materially alter the r6ntgenogram. Although skull changes are shown by the X-ray in 45 per cent. of our cases and are frequently pathognomonic, on the whole they represent late stages of the disease. As intracranial tumors come to be diagnosed and localized earlier, the value of the X-ray will be correspondingly diminished. For some time I have considered the possibility of filling the cerebral ventricles with a medium that will produce a shadow in the radiogram. If this could be done, an accurate outline of the cerebral ventricles could be photographed with X-rays, and since most neoplasms either directly or indirectly modify the size or shape of the ventricles, we should then possess an early and accurate ai-d to the localization of intracranial affections. In addition to its radiographic properties, any substance injected into the ventricles must satisfy two very rigid exactions: (I) It must be absolutely non-irritating and non-toxic; and (2) it must be readily absorbed and excreted. The various solutions and suspensions used in pyelography-thorium,