Harris was encouraged in entomology from a young age by his uncle, a member of the Society of the Aurelians. In 1762 he became secretary of a second Society of Aurelians. He was a skilled artist, displaying some of his insect drawings at the Royal Academy in 1785. He drew and engraved illustrations for books including Dru Drury'sIllustrations of Natural History and John Coakley Lettsom's The Naturalist's and Traveller's Companion.
Colours
In the Natural System of Colours he examined the work of Isaac Newton and tried to reveal the multitude of colours which can be created from three basic ones. Natural System of Colours was published again in 1811, this time edited by Thomas Martyn and dedicated to the second President of the Royal Academy, Benjamin West. As a naturalist, Harris wished to understand the relationships between the colours, and how they are coded, and his book attempted to explain the principles, "materially, or by the painters art", by which further colours can be produced from red, yellow and blue. Harris showed what is now known as the subtractive mixing of colours, observing that black is formed by superimposition of the three basic colours.
Entomology
Harris published his The Aurelian or natural history of English insects in 1766. In 1780 he followed this up with the first scientific descriptions of several Odonata including the banded demoiselle, Calopteryx splendens, and was the first English artist to make illustrations of dragonflies accurate enough to be identified to species. Reviewing his artwork, the odonatologists Albert Orr and Matti Hämäläinen comment that his drawing of a 'large brown' was "superb", while the "perfectly natural colours of the eyes indicate that Harris had examined living individuals of these aeshnids and either coloured the printed copper plates himself or supervised the colourists." However, they consider the larva on the same plate far less good, "a very stiff dorso-lateral view of an aeshnid larva with mask extended. No attempt has been made to depict the eyes, antennae or hinge on the mask or labial palps, all inconceivable omissions for an artist of Harris' talent had he actually examined a specimen", and they suggest he copied it from August Johann Rösel von Rosenhof. In 1778, Harris discovered the Muscina sp.|Muscina levida species of fly. Two years later, he followed with a discovery of the Muscina prolapsa species of fly.
