We present two conceptual optical designs for a new refractive corrector for the prime focus of the 4.2m William Herschel Telescope, optimised to allow wide-field multi-object spectroscopy. The proposed designs satisfy the demanding requirement that the PSF be smaller than 0.5 arcsec (80% encircled energy) over a two degree FOV and a wavelength range of 370 - 1000 nm. We discuss the specifications and describe the design process for the correctors, which also act as atmospheric dispersion correctors (ADC). The designs we present form the basis of a realistic manufacturable system.