Oliver Byrne, the first six books of the Elements of Euclid, in which coloured diagrams and symbols are used instead of letters for the greater ease of learners

Euclid's geometry - (not) a Via Regia? = Eiklids Geometrie - (k)ein Königsweg? = La géométrie d'Euclide - une voie royale?

"To facilitate their acquirement": Oliver Byrne's The first six books of the elements of Euclid - didactic, colourful, and eccentric = didaktisch, farbig und exzentrisch = didactique, coloré et excentrique.