Career
He is also well known for Harker–Kasper inequalities which he devised in collaboration with John South. Kasper. Harker made seminal discoveries in the field of chemical crystallography. His lab solved the structure of the pancreatic enzyme ribonuclease A, the third protein structure ever solved by protein crystallography.
After retirement from RPCI in 1976, he joined the Hauptman-Woodward Medical Research Institute (HWI), then known as the Medical Foundation of Buffalo.
He remained there until his death in 1991. His research interests while at HWI turned towards mathematical aspects of crystallography, including color space groups and infinite polyhedra.