This study concerns two scientific texts of the late seventeenth century. One is Newton’s Opticks, written in English; the other, written in French, is Traité de la lumière by Huygens. It is argued that the two writers have different ideologies, in the sense that Newton is working in an empirical framework, whereas Huygens is a Cartesian. This has an effect on the semantics and lexicogrammar of the two texts. The percentage of Material process passives, the type of personal pronouns and the process types with which they are used, and the types of processes which are nominalized are all features which reflect the empirical stance of Newton as opposed to the Cartesian position adopted by Huygens. The contexts of the two texts are virtually identical, measured in terms of Field, Tenor and Mode. This has implications for SFL theory, and allows for the hypothesis that ideology affects the semantic metafunctions directly.