need a paper before 1988

Daniel H. Schaubert, Donald R. Wilton, and Allen W. Glisson,
"Atetrahedral Modeling Method for Electromagnetic Scattering by Arbitrarily Shaped Inhomogenous Dielectric Bodies" , IEEE, TRANS.vol. AP-32, NO. 1, January 1984.

This is what you want!
https://www.edaboard.com/viewtopic.php?p=576939#576939
Good luck!