  With its three independent parameters, the $ABCD$ matrix serves as the beam
transfer matrix in optics. If it is transformed to an equi-diagonal form, the
matrix has only two independent parameters determined by optical devices. It is
shown that this two-parameter mathematical device contains enough information
to describe the basic space-time symmetry of elementary particles. If its trace
is smaller than two, this matrix can represent the internal space-time symmetry
of massive particles. If equal to two, the matrix is of massless particles. If
the trace is greater than two, this matrix describes imaginary-mass particles.
This matrix speaks Einstein's language for space-time structure of elementary
particles. As for the optical devices, the laser cavity and the multilayer
system are discussed as illustrative physical examples.
