The Lorentz-Formulae and the Metrical Principle

The Lorentz-formulae are deduced from three factual statements the physical meaning of which is explained in terms of operations with clocks, light-signals and measuring rods. These statements are: (1) The time-length of a process is invariant. (2) The velocity of light is the same in all inertial systems. (3) The velocity of light is independent of the source. It is also shown that these statements can be deduced from the Lorentz-formulae. They are the physical content of the latter. The principle of relativity and the light-principle together, however, contain more physical meaning than the Lorentz-formulae. Operational definitions are given for all relevant concepts.