The macroscopic method is introduced for practical purposes rather than as a contribution to the theory, and there seems to be no advantage in developing it further here. The chief theoretical interest lies in the suggestion of a possible generalisation of Maxwell's theory by admitting that the covariant and contravariant electromagnetic tensors may in certain circumstances be independent tensors, e.g.\ inside the electron. This is the basis of a theory of matter developed by G.~Mie. \PageSep{196} \Chapter{VII}{World Geometry} \Part{I.}{Weyl's Theory} \Section{83.}{Natural geometry and world geometry} \index{Natural coordinates!geometry}% Graphical representation is a device commonly employed in dealing with \index{Graphical representation}% all kinds of physical quantities. It is most often used when we wish to set before ourselves a mass of information in such a way that the eye can take it in at a glance; but this is not the only use. We do not always draw the graphs on a sheet of paper; the method is also serviceable when the representation is in a conceptual mathematical space of any number of dimensions and possibly non-Euclidean geometry. One great advantage is that when the graphical representation has been made, an extensive geometrical nomenclature becomes available for description---straight line, gradient, curvature, etc.---and a self-explanatory nomenclature is a considerable aid in discussing an abstruse subject. It is therefore reasonable to seek enlightenment by giving a graphical representation to all the physical quantities with which we have to deal. In this way physics becomes geometrised. But graphical representation does not assume any hypothesis as to the ultimate nature of the quantities represented. The possibility of exhibiting the whole world of physics in a unified geometrical representation is a test not of the nature of the world but of the ingenuity of the mathematician.