David Gill, Encyclopædia Britannica, Ninth Edition (1888)
“ The focal lengths of the two lenses which secure the conditions of achromatism having been thus computed, the radii of curvature may be computed for either lens by the usual formula (see Light, vol. xiv. p. 593) — In the last expression, where r and s correspond to the radii of curvature, the optician has an infinite range of choice. ”
