The Natural System of Colours, Wherein is displayed the regular and beautiful Order and Arrangement, Arising from the Three Premitives, Red, Blue, and Yellow, The manner in which each Colour is formed, and its Composition, The dependance they have on each other, and by their Harmonious Connections Are produced the Teints, or Colours, of every Object in the Creation, And those Teints, tho' so numerous as 660, are all comprised in Thirty Three Terms, only By Moses Harris, Author of the Aurelian, &c. &c.

No publication-date is given. But Harris died ca. 1788; the book's dedication to Sir Joshua Reynolds, President of the Royal Academy, indicates a publication-date no earlier than May 1769, when Reynolds received his knighthood; and W. Spillmann states that 'a color circle derived from the Natural System' appears in Harris's Exposition of English insects, which was published in 1776. Accordingly we should assign the present work to some date between 1769 and 1776. (There are no other firm indications of date. There is evidence that the printer George Laidler worked from 13 Prince's Street from 1775 to 1800 - but this does not firmly preclude a pre-1775 date. John Gage has surmised that since the title-page does not mention Harris's Exposition of English insects, published in 1776, it may well have been published before that date - but this, also, is uncertain. A publication-date commonly assigned to the work, that of 1766, appears to have been first mentioned in W.B. Sarsfield Taylor's translation of J.F.L. Mérimée's The art of painting in oil and in fresco (1839); but in view of the dedication this must be mistaken. Taylor does claim to have seen Harris's original manuscript, and 1766 is perhaps not implausible as a date of the book's composition.)

The work was intended to be of practical use to artists - as its dedication to the President of the Royal Academy confirms. Harris here presents colour spectra in the form of a wheel - an idea of which he is said to have been the inventor.

Each of the first two plates shows a colour circle - one captioned 'Prismatic', in which the colour range is derived directly from three primaries (red, yellow and blue), and the other 'Compound', in which it progresses from three combinations of primaries (i.e. orange, green and purple). The third plate, captioned 'Examples', is referred to in Harris's text (p. 4, ' examples, fig. 9. At the end of this book.'), but is not present in surviving copies, apart from that of W. Spillmann. The plates are hand-coloured etchings.

The in-text diagram on p.4 is a hand-coloured visual register demonstrating what is meant by the terms used to describe these colours together with a verbal one relating them to 'some known substance, fruit or flower', viz.: 'Red - Vermillion, Wild-poppy. Yellow - Kings Yellow, Butter-flower, or Meadow-renunculus. Blue - Ultra Marine, Cornbottle-flower. Orange - Red Orpiment, Garden Marigold. Green - sap-green, Leaves of the Lime-Tree. Purple - Hairy Sheeps-scabious, or the flower of the common Judas-Tree.'

The book appears to have been well received. A second edition, edited by Thomas Martyn, was published in 1811. In 1817 G. Field refers briefly to it in his Chromatics; and in 1829 Thomas Phillips referrred extensively to the work in his eighth Royal Academy lecture, 'On Colouring' (subsequently published in 1833). Yet in 1839 Taylor (who quotes from Phillips) writes 'This work of Mr. Harris is so very scarce, that I have never seen a copy of it'. Etty's presenting a copy of the first edition to the Academy in 1841 in a specially commissioned binding reflects his awareness of the book's importance, but also, perhaps, of its rarity. Of the first edition copies are certainly now extremely rare. In addition to the Royal Academy's copy the only others known are that of the Yale Center for British Art, New Haven (the Birren copy), that of the Bayerische Staatsbibliothek, Munich, and that of Mr. W. Spillmann.