Page:The principle of relativity (1920).djvu/79

 which have not at all the suffix 4, or have it twice denote real quantities.

An individual system of values of (x, y, z, t) i. e., of (x_{1} x_{2} x_{3} x_{4}) shall be called a space-time point.

Further let u denote the velocity vector of matter, ε the dielectric constant, μ the magnetic permeability, σ the conductivity of matter, while ρ denotes the density of electricity in space, and x the vector of "Electric Current" which we shall some across in §7 and §8.