This volume contains the important work which the late Professor H. A. Lorentz did to develop Maxwell's theory in the years before his middle age. It begins with the treatise concerning the relation be­ tween the propagation of light and the density of media. The Lorenz­ Lorentz formula is arrived at (page 57) and an electromagnetic dispersion theory of light is developed (p. 70, Bo) for the first time, so far as we know. This work was finished as early as I878: in an abridged form the results were published in Wiedemann's An­ nalen der Physik und Chemie (9, 64I, I88o); until now the com­ plete treatise was only accessible in the Dutch language. In the following dozen years the development of Maxwell's theory was carried further and the result was the paper published in I892, which is found in the end of the volume. The work of Boltz­ mann and Hertz dates from the same period. Lorentz took the view that matter was perfectly permeable to the ether and that the latter was invariably at rest. Still, after Hertz published his theory based on the idea that the ether contained in moving bodies completely shared the motion, Lorentz inserted a chapter developing the inferences to be drawn from this axiom (p. 206) which was contradictory to the line he followed himself in order to account for Fresnel's coefficient.