  The paper discusses the role played by Mueller and Jones formalisms in
polarization optics, by addressing the following aspects: restriction to the
SU(2) symmetry, non-relativistic Stokes 3-vectors; Cartan 2-spinors in
polarization optics; Jones 4-spinors for partially polarized light; the linear
group SL(4,R) and the classification of 1-parametric Mueller matrices;
semi-group structure and classification of degenerate Mueller matrices.
