  The formalism for the spin interactions in the front form (light-cone) is
re-phrased in terms of an instant form formalism. It is shown how to unitarily
transform the Brodsky-Lepage spinors to Bj{\o}rken-Drell spinors and to
re-phrase the so called spinor matrix in terms of the interactions one is
familiar with from atomic and Dirac theory. -- One retrieves the (relativistic)
kinetic correction, the hyperfine and the Darwin term which acts even when wave
function is spherically symmetric. One also retrieves angular momentum
dependent terms like the spin-orbit interaction in a relativistically correct
way; and one obtains additional terms which thus far have not been reported
particularly various $\vec L^2$-dependent terms. Since the approach includes
the full retardation, one gets additional, thus far unknown terms. The differ
from atomic and Dirac theory, since there only that part of the vector
potential is usually included which is generated by the atomic nucleus. Quite
on purpose, the paper is kept formal.
