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The Philosophy of Vacuum Part 5. Physicists will find it extremely interesting, covering, as it does, technical subjects in an accessible way. For those with the necessary expertise, this book will provide an illuminating and authoritative exposition of a many-sided subject." -John D. Barrow, Times Literary Supplement. | Reflections on Ether 33 forces all these things being entirely unknown. Rather is it expedient to start from these equations in search of such further conjectures respecting the constitution of the ether. Hertz 1893 201 emphasis ours. 4 This was essentially Einstein s perspective on Maxwell s theory with one important difference there could be no ether for there could be no absolute resting frame. Were there then no further conjectures that one might seek to make concerning the constitution not of the ether but of radiation Was Maxwell theory complete Return to Lorentz. Already in his Ph.D. thesis that is in 1875 he had sharply distinguished the polarization of molecules from the polarization of the ether. This he did in order to explain the fact that the dielectric constants and refractive indices of gases are always close to unity. He made the simple assumption that the dominant electromagnetic contribution comes from the ether and not from the molecules of a dilute system. Indeed if the inductive capacity of the molecules is proportional to their number density N so that e0 otN then from the expression n2 l 4ue l 4ir 0 where n is the refractive index and assuming the density p is proportional to N there follows the law of Arago and Biot n2 l p constant. In particular as p- 0 it follows that n- l. Lorentz speculated that quite generally one should take into account first the ether and then the molecules lying within it. Then the distance size and form of the latter enter into consideration from which the explanation of dispersion and the plane of polarization will probably result. 5 The theory of dispersion which followed three years later amply fulfilled his expectations. The basic mechanism for the dispersion was in fact already known the essential point is to treat each molecule within the dielectric as elastically bound but driven by the incident wave. Maxwell and Sellmeier independently hit on this mechanism in the context of the elastic solid ether Lorentz
