An unpublished preface to Gregory's Optics. He praises Gregory's language, and excuses his editorial limitation to spherical surfaces. Since Descartes' discovery of the sine law of refraction, he says, optical writers have expected too much from still-crude contemporary reflective surfaces.

Charles Oliphant is known to us only as a physician, not a scientist, but may also have been Gregory's brother-in-law; he may be the same Charles Oliphant as the one Gregory taught briefly at Edinburgh, who graduated with a medical degree in 1684.