Mary Gartside’s An Essay on a New Theory of Colours is the expanded edition of An Essay on Light and Shade (1805), “one of the rarest and most unusual books about colour ever published”, claims Alexandra Loske, a historian of colour and curator at Brighton’s Royal Pavilion. On the surface, this tract appears to sit neatly within the tradition of instructional artist’s manuals. Indeed, Mary Gartside worked as a watercolour teacher and botanical painter, exhibiting her drawings at the Royal Academy in 1781. And yet, the Essay makes use of an intellectual palette whose spectrum exceeds Gartside’s pedagogical contemporaries. It is best remembered, rather, as an exemplar of the myriad early-nineteenth century treatises on colour — works inspired, in part, by the newfound availability of novel pigments.

Gartside’s Essay is largely concerned with those gradations between the primary colours, appealing to both students and “philosophical readers” through its discussion of tint, warmth, transformation, and luminance. Referencing Isaac Newton’s experiments with prismatic refraction (picture Pink Floyd’s album art for The Dark Side of the Moon), Gartside explains the phenomenon to “young ladies who have not seen [the Prismatic Spectrum]”, while offering analysis and critique of eighteenth-century theories proposed by Gerard de Lairesse and William Herschel, namely the latter’s investigation into the order of colours. Not only did Gartside antedate James Sowerby’s A New Elucidation of Colours (1809) and Goethe’s Theory of Colours (1810) — drawing parallel conclusions about “the effect of colour combinations, the significance of light and shade in relation to tints, and the eye of the beholder as the centre and origin of colour perception” — the handcoloured illustrations for the Essay, unique to each volume, have been deemed some of the earlier examples of abstraction in painting.

We know very little about Gartside’s biography. According to Loske, she appears to be one of the only nineteenth-century women to have composed “theoretical treatises on colour”, nearly a century before Emily Noyes Vanderpoel published her Color Problems (1902). “The very modesty of the genre”, writes Ann Bermingham, in reference to why the theorist had to disguise her optical treatise as a how-to guide, “obscured the originality of [her] inquiries”, which, in turn, allowed Gartside to pursue scientific matters without reproach. Read in this light, one footnote in the Essay becomes particularly illuminating:

. . . though I know the old masters took the rainbow for their guide, the little opportunity I have had of seeing their works, has prevented me observing how they availed themselves of the lessons it afforded them. They also possessed a degree of philosophical knowledge, that made them profit more speedily from their observations than any one can do without that knowledge; who must ignorantly copy nature, till some scientific person shall point out the way to do it to advantage.

Yet if Gartside did, in fact, “copy nature” in her floral work, it was not out of ignorance, and the results were anything but derivative. Resembling hydrangeas refracted through an otherworldly prism, the watercolour illustrations that accompany her Essay — featured below — are described by the artist as “compact blots of colours”, which, placed before the beholder’s imaginative eye, are capable of conjuring white dog roses, scarlet poppies, and wild hops, “clinging with graceful wreaths and festoons” to evergreen shrubs.