Page:The principle of relativity (1920).djvu/246

 and we get finally:—

[part][function]_{1 2}/[part]x_{2} + [part][function]_{1 3}/[part]x_{3} + [part][function]_{1 4}/[part]x_{4} = ρ_{1} } [part][function]_{2 1}/[part]x_{1} + [part][function]_{2 3}/[part]x_{3} + [part][function]_{2 4}/[part]x_{4} = ρ_{2} }  (3)  [part][function]_{3 1}/[part]x_{1} + [part][function]_{3 2}/[part]x_{2} + [part][function]_{3 4}/[part]x_{4} = ρ_{3} }  [part][function]_{4 1}/[part]x_{1} + [part][function]_{4 2}/[part]x_{2} + [part][function]_{4 3}/[part]x_{3} = ρ_{4} }  [P. C. M.]  Note 9. On the Constancy of the Velocity of Light.

Page 12—refer also to page 6, of Einstein's paper.

One of the two fundamental Postulates of the Principle of Relativity is that the velocity of light should remain constant whether the source is moving or stationary. It follows that even if a radiant source S move with a velocity u, it should always remain the centre of spherical waves expanding outwards with velocity c.

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.