The word rectilinear simply means " right lines," and is a name applied to lenses which do not distort straight lines when such fall near the margins of the plate. Such lenses represent a square as a square, and not like a pin-cushion or a barrel, as is the case with a single lens when the stop is placed respectively behind or before the lens. Consequently, rectilinear lenses are doublets - that is, they have a lens at each end of a tube, with the stop between, thus introducing both kinds of distortion, the one nullifying the other.