The posterior median eyes of Dinopis subrufus are probably the largest simple eyes found in arthropods, and may reach 1.40 mm in diameter. For a lens 1.325 mm in diameter the focal length is 0.771 mm, giving an F-number of 0.58. This, coupled with the large diameter of the receptors (20 $\mu $m), means that the light absorbed per receptor will be about 2000 times as great as in a diurnal spider such as a salticid, or in the human eye looking at the same extended field. For a lens of this size to produce a resolution of 1.5 degrees - the visual subtense of each receptor - it is shown that spherical aberration has been overcome. This is achieved partly by the lens having a double structure, with an outer region of low refractive index, and a harder core whose apparent refractive index is 1.67. It is argued from the observed resolution of the lens that the core is probably inhomogeneous, like a fish lens, with a central refractive index that could be as low as 1.52. The hemispherical eye-capsule is modified to bring the retina close to the rear surface of the lens, a feature necessitated by the relatively short focal length. The large, hexagonal receptive segments are tightly packed, but there is no tapetum. Some retinal anomalies, the structure of the other eyes of Dinopis, and the relation of its visual optics to its nocturnal habit are also discussed.