The result of the experiments is absolutely clear and consistent, and some of them have dealt with Beta rays corresponding to a mass-value ten times greater than the original mass. This result is: mass varies with velocity, and in exact accord with the numerical laws of Einstein’s dynamics. Here is a new and valuable experimental confirmation. This in turn tends to show that classical mechanics was merely a rough approximation, valid at the most only for the comparatively slight velocities with which we have to deal in the very restricted course of daily life. Thus the mass of bodies, the Newtonian property which was believed to be the very symbol of constancy, the equivalent of what loyalty to treaties is in the moral order of things, is now merely a small coefficient, variable, undulating, and relative to the point of view. In virtue of the reciprocity which we have described, when there is question of contraction due to velocity, the mass of an object increases in the same way, not only if the object is displaced, but if the observer is displaced, and without any other observer, connected with the object, being able to detect the difference. For instance, a measuring rod that moves at a velocity of about 260,000 kilometres a second will not only have its length shortened by one-half, but will have its mass doubled at the same time. Hence its density, which is the relation of its mass to its volume, will be quadrupled. The physical ideas which were believed to be most solidly established, most constant, most unshakeable, have been uprooted by the storm of the new mechanics. They have become soft and plastic things moulded by velocity. * * * * * Further confirmations of the new formula, quite independent of the one we have just described, have recently been provided by physicists. One of the most astonishing of these is given in spectroscopy. As is well known, when we cause a ray of sunlight, admitted through a narrow slit, to pass through the edge of a glass prism, the ray expands, as it issues from the prism, like a beautiful fan, the successive blades of which consist of the different colours of the rainbow. When we examine closely this coloured fan, we notice certain fine discontinuities, narrow lines or gaps, in which there is no light. They look like cuts made with a pair of scissors in our polychrome fan. They are the dark lines of the solar spectrum. Each of these lines, or each group of them, corresponds to a special chemical element, and serves to identify this, whether in our laboratories or in the sun and the stars.