APPENDIX 2. CALCULATIONS IN CONNEXION WITH ETHER DENSITY 146 APPENDIX 3. FRESNEL'S LAW A SPECIAL CASE OF A UNIVERSAL POTENTIAL FUNCTION 152 LIST OF ILLUSTRATIONS FIG. _Illustrations of Aberration._ 1. Cannon shots 35 2. Boats or Waves 36 3. Lighthouse beams 37 4. Ray through a moving stratum 40 5. Wave-fronts in moving medium 41 6. Normal reflexion in moving medium 43 _Experiments on Ether drift._ 7. Interference Kaleidoscope 51 8. Hoek's experiment 53 9. Experiment of Mascart and Jamin 54 10. Diagram of Michelson's experiment 61 _Illustrations of Ether Machine_ (Lodge). 11. Diagram of course of light 69 12. General view of whirling part of Ether Machine 72 13. General view of optical frame 73 14. Drawing of optical details 74 15. View of Ether Machine in action _Frontispiece_ 16. Appearance of interference bands and micrometer wires 76 17. Iron mass for magnetisation 77 18. Appearance of bands 76 19. Arrangement for electrification 78 INTRODUCTION "Ether or Æther (αιθηρ probably from αιθω I burn,) a material substance of a more subtle kind than visible bodies, supposed to exist in those parts of space which are apparently empty." So begins the article "Ether," written for the ninth edition of the _Encyclopædia Britannica_, by James Clerk Maxwell. The derivation of the word seems to indicate some connexion in men's minds with the idea of Fire: the other three "elements," Earth, Water, Air, representing the solid, liquid, and gaseous conditions of ordinary matter respectively. The name Æther suggests a far more subtle or penetrating and ultra-material kind of substance. Newton employs the term for the medium which fills space--not only space which appears to be empty, but space also which appears to be full; for the luminiferous ether must undoubtedly penetrate between the atoms--must exist in the pores so to speak--of every transparent substance, else light could not travel through it. The following is an extract from Newton's surmises concerning this medium:--