Fig. 1. Concept of the multi-immersion Schmidt objective.

a, Our approach is inspired by the anatomy of the scallop eye. In this eye design, a curved mirror in contact with a liquid forms an image on a transparent layer of photoreceptors. b, A second inspiration is the Schmidt telescope, a mirror-based wide-field telescope design that consists of a spherical mirror and an aspherical correction plate. c, We synthesize both concepts into a multi-immersion objective design that provides a sharp image in any homogeneous medium (center). Reflection off the mirror does not depend on the refractive index n of the immersion medium (right). To avoid additional refraction at the inner surface of the correction plate, we deform this surface such that all passing rays are close to normal incidence (left).