Prof.\ Weyl himself has come to prefer the second alternative. He draws a \index{Adjustment and persistence}% \index{Persistence and adjustment}% useful distinction between magnitudes which are determined by \emph{persistence} (\Foreign{Beharrung}) and by \emph{adjustment} (\Foreign{Einstellung}); and concludes that the dimensions of material objects are determined by adjustment. The size of an electron is determined by adjustment in proportion to the radius of curvature of the world, and not by persistence of anything in its past history. This is the view taken in \SecRef{66}, and we have seen that it has great value in affording an explanation of Einstein's law of gravitation. The generalised theory of Part~II leads almost inevitably to the second alternative. The first form of the theory has died rather from inanition than by direct disproof; it ceases to offer temptation when the problem is approached from a broader point of view. It now seems an unnecessary speculation to introduce small ambiguities of length-comparisons too small to be practically detected, merely to afford the satisfaction of geometrising the vector~$\kappa_{\mu}$ which has more important manifestations. The new view entirely alters the status of Weyl's theory. Indeed it is no \index{Weyl's theory!modified view of}% longer a hypothesis, but a graphical representation of the facts, and its value lies in the insight suggested by this graphical representation. We need not now hesitate for a moment over the identification of the electromagnetic potential with the geometrical vector~$\kappa_{\mu}$; the geometrical vector is the potential because that is the way in which we choose to represent the potential graphically. We take a conceptual space obeying Weyl's geometry and represent in it the gravitational potential by the $g_{\mu\nu}$~for that space and the electromagnetic potential by the $\kappa_{\mu}$~for that space. We find that all other quantities concerned in physics are now represented by more or less simple geometrical magnitudes in that space, and the whole picture enables us to grasp in a comprehensive way the relations of physical quantities, and more particularly those reactions in which both electromagnetic and mechanical variables are involved. Parallel displacement of a vector in this space is a definite operation, and may in certain cases have an immediate physical interpretation; thus when an uncharged particle moves freely in a geodesic its velocity-vector is carried along by parallel displacement~\Eq{(33.4)}; but when a material measuring-rod is moved the operation is not one of parallel displacement, and must be described in different geometrical terms, which have reference to the natural gauging-equation~\Eq{(89.1)}. \PageSep{209}