1. STRAUBEL , R. 1927. Gehrcke's Handbuch der Physikalischen Optik Vol. I, 233Leipzig see
2. LUNEBERG , R. K. 1944. Mathematical Theory of Optics 213Providence
3. LUNEBERG , R. K. 57 l. c, BORTOLOTTI'S priority, which was unknown to LUNEBERG, was kindly pointed out to the author by Professor D. GRAFFI in a private communication