"For almost a century it has been the common belief that light is merely a wave motion in the ether. Our eyes respond to ether waves of certain length only. Waves a little longer than those which affect our eyes are felt by us as heat waves. Waves still longer than those of heat are the so-called electric waves. These we use in wireless telegraphy. There are still shorter waves than those of light. These affect the sensitive plate in photography. They help to form the green material in the leaves of plants and the brilliant colours in flowers. They assist in the fading of our clothes and the tanning of our skin. These are called chemical waves. Still shorter waves in the ether than those of which we have just spoken are the X rays, and all the strange things which they may do have not yet been determined. Certain it is that they can make dreadful sores in our flesh. They can penetrate through wood and paper, but not metals. They pass readily through flesh, but not bones. All such ether waves are treated in a book by Sylvanus P. Thompson, entitled 'Light Visible and Invisible,' in which he points out that electricity, heat, light, chemical rays, etc., are all alike in being ether waves, and this was suspected by James Clerk Maxwell and others half a century ago, and has come now to be quite generally believed. "Halley's comet, already having been _seen_ upon this return, must be sending out those ether waves which we call light; whether it is also sending forth some of the other kinds of ether waves may yet be determined." My audience being chiefly composed of those persons who were present at Harold's birthday party, they pressed me to tell them more about wireless telegraphy and similar matters, and so I agreed to give them at some future date some account of the history of these ideas. But my present purpose was to start an interest in astronomy as an antidote for the wireless epidemic, and so I invited all who desired to do so to come again one week from that evening, bringing with them such opera and field glasses as they might be able to secure. I promised to show them how to make a telescope such as Galileo had more than three hundred years ago. I agreed to go out with them several evenings and scan the sky with our telescopes, and to tell them of some readable books and articles upon astronomical matters. XXV HOW THE IDEA OF A UNIVERSAL ETHER DEVELOPED