  The special theory of relativity is constructed demanding the retention of
the rectilinear form of a trajectory and invariance of the wave equation under
linear transformations of space and time coordinates. The usual approach to
relativity based on manipulations with the impulse of light is shown to be
owing to that the symmetry of the particular solution of the wave equation
coincides with the symmetry of the very wave equation. Thereof instead of the
equation in partial derivatives we may deal with the algebraic form referred to
as the interval.
