When an image of the anode of an x-ray tube is obtained on a film placed before a lead diaphragm having an opening 1 mm. in diameter, all parts of the anode are reproduced, as shown in Fig. 1, thus indicating that they all emit x-rays. In the figure, zone A corresponds to that part of the tungsten target which receives the greatest bombardment of electrons; zone B represents the remainder of the tungsten target, and zone C the copper support, or anode stem. The intensity and quality of the radiation emitted by each of these zones have been studied by three methods: (1) independent micro-ionization chambers; (2) the Strauss dosimeter; (3) blackening of an x-ray film. (1) For the purposes of this study an ionization chamber measuring but a few millimeters in diameter was constructed. For the central insulated electrode and the outer electrode a substance of low atomic weight, as magnesium or some organic product, was used. The charge of the central electrode and the charge remaining after exposure are measured by means of a special electrometer providing an automatic charge, called by us “radiometre.” (2) The Strauss dosimeter measurements were made in accordance with the usual technic. (3) The third method consisted in comparing the degree of blackening of an x-ray film with the aid of a densitometer, a standard scale of tints being used for quantitative determinations. With the micro-ionization chambers, it was found that at 100 kv. the greater the diameter of the aperture in the lead diaphragm, the more penetrating was the radiation. This observation prompted us to investigate the influence of the different parts of the anode upon the quality of the radiation. For this purpose the pin-hole diaphragm used to obtain the image of the anode was employed, the ionization chambers being placed successively in each of the three zones, A, B, and C. As will be explained more fully below, it was found that the radiation emitted by the central zone, A, was of the greatest wave length, while the most penetrating rays were those from zones B and C. These results were confirmed by densitometer determinations. The experiment was completed by a study of the effect of distance upon the quality of the radiation. It was found that with a large aperture in the limiting diaphragm, the radiation was most penetrating at the point of exit of the tube. Details of Experimental Studies (1) Determinations with Micro-Ionization Chambers: With the specially constructed ionization chambers, the half-value layer of the radiation from the different zones, when the distance between the anode and the ionization chamber was 16 inches, was found to be as follows: The intensity of the radiation from zone A was 220 times that from B, while the intensity of the radiation from B was twice that from C. It does not follow, however, that zone B is responsible for only 1/220 and zone C for only 1/440 of the total radiation, since zones B and C are much larger than A.