The 4 metre, F/2.8, Ritchey-Chretien (R-C) telescope of Kitt Peak National Observatory (KPNO) is equipped with a "grism" (grating+prism) located after the 3-element Wynne R-C corrector to provide slitless spectra which are, unfortunately, restricted to a 30 arcmin field at the prime focus although its corrector is designed for a 50 arcmin field. The performance could be improved if the grating were located a maximum distance from the focal surface, preferably on the last element of the corrector: a grating-on-lens or "grens". The last surface of the corrector lens should be made flat if it is to be used as a blank for a transmission grating. Optical parameters are given for such a corrector having a slightly larger diameter but with a flat last surface for a 4 metre, F/2.8, R-C telescope. For use as a grens this third element must be interchanged with a wedged element with the transmission grating deposited thereon. Compared with the grism, the field increases to 50 arcmin with the grens, the spectral resolution improves at the extremes of the wavelength region and the transmission increases because only three instead of five elements are used. Even better performance could be achieved if, instead of being R-C, the telescope were Classical because the field could be increased to 1 degree and the spectral resolution improved with a modestly enlarged corrctor the diameter of which need not be larger than the modified Wynne Classical corrector of the 3.6 metre, Classical, F/3.8, Canada France Hawaii Telescope (CFHT). By comparison, enlargement of the Wynne R-C corrector decreases the resolution. However, a new corrector design for R-C telescopes can equal the Classical performance but requires an unprecedentedly large triplet lens.