Abstract. In designing a fused bifocal spectacle lens we have at our disposal three radii which can be varied to satisfy the two conditions, that the distance power and the reading addition have given values, so leaving one degree of freedom.

An expression is found for the convergence of the circles of least confusion of oblique pencils of small aperture; from this, by equating the aberration term to zero, we obtain a quadratic equation the roots of which fix the radius of one surface of the lens subject to the third condition that the circles of least confusion lie on a sphere and so focus on the macula of the eye.

Print publication: Issue 3 (1930)
Received 20 February 1931