E. T. Whittaker: Proc. London Mathematical Soc., Series 2, Vol. 1, p. 367-372 (1904) ~ Page 368
"On An Expression of the Electromagnetic Field Due to Electrons by Means of Two Scalar Potential Functions"
[ Note: The equation in the middle of page 368 appears to be in error in the original. The "c-square, del-square, a-sub y term is missing the superscript 2 on the del in the original paper. From looking at the other equations in the group, it seems like it should be squared also. -- B. Paddock ]