The tensor analytical relationships of Dirac's equation
  Dirac's equation of the electron will be discussed by using quaternions as the basis of a new formalism which seems to be very well adapted to the problem. The transformation properties of the equations as well as the invariant and covariant [bilinear] constructions of Dirac's theory are developed uniformly and systematically. A method of obtaining a covariant formulation of the equations using customary tensor calculus also offers itself unequivocally if we duplicate the Dirac equations. (Editorial note: In this paper Lanczos introduces his ``fundamental equation,'' Eq. (54), from which two Dirac fields forming an isospin doublet, or two spin 1 fields of opposite parities, derive.) 
Cornelius Lanczos