This paper presents in brief the results of examination, analysis and documentary research into the grounds, medium and pigments of 17 British paintings dated 1848-60, by Brown, Hunt, Hughes and Millais, and identifies the extensive use of transparent pigments, mediums of high refractive index and references to copal. It describes the findings from practical experiments in the use of copal-based medium and varnishes, in combination with transparent pigments in oil. This allows an extensive discussion of the optical principles underlying Pre-Raphaelite methods, in relation to the analytical findings.