This paper presents a theory of transparent gravitational lenses in the Newtonian limit (the perturbations of the gravitational field by mass inhomogeneities in the universe are assumed weak both on the cosmological and on local scales). In a partially evacuated cosmic "bottle" the light-ray trajectory is derived in a simple manner, suitable for use in a lens theory. The theory is presented in a simple form, applicable to single-redshift as well as to multiredshift lenses. The simplified case of a "nonlinear telescope", composed of many elements at different distances, is analyzed in detail. The universal properties of caustics of an almost spherical lens with a weak quadrupole (tidal) component are analyzed.