Modern Lens Design, 2nd Edition

^*The symmetric two-meniscus periscopic lens consists of a pair of landscape lenses (Sec. 2.4) oriented with their concave surfaces toward a central stop. Despite its superficial resemblance to the Hypergon, the periscopic lens cannot be considered to belong in this class of lenses. The elements are relatively thin, so that there is only a tiny effect on the Petzval sum (which is completely offset by the fact that there are two positive elements spaced apart from each other). In the Hypergon the two surfaces have nearly equal radii so their Petzval contributions almost completely offset each other, and the power of the element derives from the relatively large thickness separating the surfaces.