michelson's recent researches on light. 467 



different methods of investigation, astronomers and physicists will be 

 slow to abandon the theory of undulations, and take up again the cor- 

 puscular theory of light. The latter theory has received fatal blows 

 from which it cannot recover. Tlie unduhitory theory, which started 

 with Huyghens more than two hundred years ago, and was elaborated 

 by Fresuel sixty years ago, has survived many crises in its history, and 

 is supported by a wonderful array of experiments. Some of the experi- 

 ments of Mr. jMichelson may require a. modification iu Fresnel's inter- 

 pretation. Stokes and Ohallis have worked for many years upon it, 

 and established it on mathematical princi[)les differing from Fresnel's 

 and from each other. Ketteler in his Thcoreti.schc Optil\ published iu 

 1885, builds upon the Sellmeier hypothesis, that ponderable particles 

 are excited by the a^therial vibrations and then re act upon them. There 

 remains Maxwell's electro-magnetic theory of light, which has been 

 elaborated by Glazebrook and Fitzgerald, and is supported, to say the 

 least of it, by remarkable numerical coincidences. 



Discrepancies between theory and experiment are always to be wel- 

 comed, as they (iontain the germs of future discoveries. We have 

 learned in astronomy not to be alarmed by them. More than once the 

 law of gravitation has been put on trial, resulting iu a new discovery 

 or iu improved mathematical analysis. We may not expect in light 

 such a brilliant discovery as that of the planet Neptune. The luminif- 

 erous ;ether is a mysterious substance, enough of a fluid for the planets 

 to pass easily through it, but at the same time enough of a solid to admit 

 of transverse vibrations. Stokes suggests water with a little glue dis- 

 solved in it as a coarse representation of what is re<]uired of the {^.ther. 



Mr. G. A. Hirn has written recently on the constitution of celestial 

 space. He decides against the existence of an ali-pervading medium. 

 He thinks that matter exists in space only iu the condition of dfstiuct 

 bodies, such as stars, planets, satellites, and meieorites. In nebulae it 

 is in a state of extreme diffusion; but elsewhere space is empty. But 

 how would it be after the correction is applied for the equation of light? 

 Humboldt said that the light of distant stars reaches us as a voice from 

 the past. Tlie astronomer is not seeing for the most part contempo- 

 raneous events. He is reading history; and often ancient history, and 

 of very different dates. Stellar photography reveals millions of stars 

 which cannot be seen in the largest telescopes, and new harvests of 

 these blossoms of heaven (as they have been called) spring up like the 

 grass in the night. Numbers fail to express their i)rol)able distances 

 and the time taken by their light in coming to the eartli. In the 

 theogony of Hesiod, the brazen anvil took only nine days iu falling 

 from heaven to earth. On the other hand, the reduction of the sun's 

 distance by three per cent, not only atlects its mass and heat, but it 

 changes the unit of measure for the universe. Such are the remote 

 results of any change iu the estimated velocity of light, 



