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Rh A. Einstein has developed this theory of relativity starting with the fundamental idea of the constancy of the velocity of light, and has thus been able to present us with a new kinematics which is apparently more consistent with the modern theories of electrodynamics than the approximate kinematics to which we are accustomed. Some of the most interesting results of the theory are that the resultant of two velocities, both of which are less than that of light, is always a velocity less than that of light; the resultant of two velocities one of which is equal to that of light is a velocity equal to that of light; the resultant of two velocities equal to that of light but of opposite directions is indeterminate, and may have any value less than or equal to that of light.

This theory of relativity is not based simply on theoretical considerations; it has received considerable support from some very delicate experiments. It was first put forward in an approximate form by Lorentz and Larmor, following a suggestion made by FitzGerald , to explain the negative results of the Michelson-Morley experiment. It was then found that it provided an ample explanation of a number of other negative results concerning the effect of the motion of the Earth on double refraction, the rotation of the plane of polarisation , the resistance of a piece of metal , and other physical phenomena. Also, the theoretical formula