The collection of technical knowledge at the service of artists desirous of externalizing their expressions, can be divided into groups, which may be entitled _theories of the arts._ Thus arises a theory of Architecture, comprising mechanical laws, information relating to the weight or resistance of the materials of construction or of fortification, manuals relating to the method of mixing lime or stucco; a theory of Sculpture, containing advice as to the instruments to be used for sculpturing the various sorts of stone, for obtaining a successful mixture of bronze, for working with the chisel, for the accurate casting of the clay or plaster model, for keeping clay damp; a theory of Painting, on the various techniques of tempera, of oil-painting, of water-colour, of pastel, on the proportions of the human body, on the laws of perspective; a theory of Oratory, with precepts as to the method of producing, of exercising and of strengthening the voice, of attitude in impersonation and gesture; a theory of Music, on the combinations and fusions of tones and sounds; and so on. Such collections of precepts abound in all literatures. And since it is impossible to say what is useful and what useless to know, books of this sort become very often a sort of encyclopædias or _catalogues of desiderata._ Vitruvius, in his treatise on Architecture, claims for the architect a knowledge of letters, of drawing, of geometry, of arithmetic, of optic, of history, of natural and moral philosophy, of jurisprudence, of medicine, of astrology, of music, and so on. Everything is worth knowing: learn the art and have done with it. [Sidenote: _Technical theories of the different arts._]