Spectacle lenses which correct oblique astigmatism at all ocular rotations: investigation of "Merté's' conicoidal surfaces.

Merté (1950) described lens forms that correct oblique astigmatism at all ocular rotations. The front surfaces of all lenses were flat, and the back surfaces were made aspherical as conicoids. Merté did not provide a derivation of his formulae for the appropriate conicoidal surfaces. We show that Merté's formula is correct and describe the nature of appropriate conicoidal back surfaces of spectacle lenses.