GAMMA radiation is emitted by a considerable number of radio-active elements which bear a close relationship to X-rays. This very complex radiation constitutes a spectrum of lines, characteristic of the discharging radio-element, the wave lengths of which vary from 0.25 to 0.005 Ångström unit. This means that the gamma rays extend the territory of X-rays to high frequencies. Their energies expressed in volts can vary from 50,000 to over two millions. The power of penetration of these radiations is, in general, far superior to that of X-rays. The effects of certain gamma rays are perceptible through 10, or even 20, cm. of lead. Thus, the coefficient of absorption in a lead screen is susceptible of falling to the value µ = 0.5 cm. for the very hard gamma rays of Radium C. Moreover, it seems that the law of variation of absorption proportionate to the cube of the wave length (law of Bragg-Peirce) is equally true for gamma radiation. In general, the X- and gamma-rays exhibit the same properties, modified in the latter, however, by the effect of the extreme shortness of the wave length. Hence the scattering of the gamma rays in material substances includes both a classic effect (Thomson) without change of wave length and a Compton effect. The quantum of scattering in this instance assumes an importance which becomes more considerable as the frequency increases, and for radiations, the energy of which corresponds to more than a half-million volts, it is just about the only one observed. The softest gamma rays exhibit very distinct diffractive effects in crystals, effects which have been utilized in the measurement of their wave length (Rutherford and Andrade, Frilley, Thibaud). The author has made use of the rotating crystal method and a spectrograph (Fig. 1), which shows the considerable development of the collimating slit system the length of which reaches 25 centimeters. Figure 2 represents the gamma spectrum of the radiothorium. On the right side the rays have energies which do not exceed the energy of X-rays. A weaker line is observed, equal to 0.052 Ångström unit, the quantum reaching 236.000 volts. However, when the frequency rises, the angles of diffraction on usual crystals (rock salt d = 2.814 Å.) do not extend beyond a few tenths of minutes and measurements of the wave lengths become altogether inaccurate. Moreover, the intensity of the lines weakens considerably, and the percentage of electromagnetic effect in the spherical coefficient of gamma-ray scattering diminishes rapidly when the quantum exceeds 200,000 volts. On the contrary, the effect of Compton scattering is particularly clear-cut: the recoil electrons acquire considerable speed. Photographed in the Wilson chamber, their trajectories present a rectilinear course, at times exceeding a decimeter, which distinguishes them clearly from the recoil electrons produced by the impact of the X-rays (fish tracks), measuring, at most, a few millimeters.