At first sight, it may not appear clear why the velocity should remain constant. Indeed according to the theory of Ritz, the velocity should become _c_ + _u_, when the source of light moves towards the observer with the velocity _u_. Prof. de Sitter has given an astronomical argument for deciding between these two divergent views. Let us suppose there is a double star of which one is revolving about the common centre of gravity in a circular orbit. Let the observer be in the plane of the orbit, at a great distance Δ. [Illustration.] The light emitted by the star when at the position A will be received by the observer after a time, Δ/(_c_ + _u_) while the light emitted by the star when at the position B will be received after a time Δ/(_c_ - _u_). Let T be the real half-period of the star. Then the observed half-period from B to A is approximately T - 2Δ_u_/_c²_ and from A to B is T + 2Δ_u_/_c²_. Now if 2_u_Δ/_c²_ be comparable to T, then it is impossible that the observations should satisfy Kepler’s Law. In most of the spectroscopic binary stars, 2_u_Δ/_c²_ are not only of the same order as T, but are mostly much larger. For example, if _u_ = 100 _km_/sec, T = 8 days, Δ/_c_ = 33 years (corresponding to an annual parallax of ·1″), then T - 2_u_Δ/_c²_ = 0. The existence of the Spectroscopic binaries, and the fact that they follow Kepler’s Law is therefore a proof that _c_ is not affected by the motion of the source. In a later memoir, replying to the criticisms of Freundlich and Günthick that an apparent eccentricity occurs in the motion proportional to _ku_Δ₀, _u₀_ being the maximum value of _u_, the velocity of light emitted being _u₀_ = _c_ + _ku_, _k_ = 0 Lorentz-Einstein _k_ = 1 Ritz. Prof. de Sitter admits the validity of the criticisms. But he remarks that an upper value of _k_ may be calculated from the observations of the double star β-Aurigae. For this star, the parallax π = ·014″, _e_ = ·005, _u₀_ = 110 _km_/sec, T = 3·96, Δ > 65 light-years, _k_ is < ·002. For an experimental proof, see a paper by C. Majorana. Phil. Mag., Vol. 35, p. 163. [M. N. S.] Note 10. Rest-density of Electricity. If ρ is the volume density in a moving system then ρ√(1 - _u²_) is the corresponding quantity in the corresponding volume in the fixed system, that is, in the system at rest, and hence it is termed the rest-density of electricity. [P. C. M.] Note 11 (page 17) Space-time vectors of the first and the second kind.