The least-squares method for optical correction, described in a previous paper is here modified slightly. An "unsophisticated" bending is used, and this permits the use of lens separations as variables. Two applications of the least-squares procedure lead to a good simultaneous correction in all four pencils used.
SAUL ROSEN and AN-MIN CHUNG, "Application of the Least-Squares Method," J. Opt. Soc. Am. 46, 223-226 (1956)