1. A reference to certain lenses in special microscopes, cameras, etc.; giving distinct vision obliquely, or all around, as well as, or instead of, in a direct line.
2. Periscopic spectacles, spectacles having concavo-convex or the convexo-concave lenses with a considerable curvature corresponding to that of the eye, to increase the distinctness of objects viewed obliquely.
3. A reference to periscopes or their uses.