2 The Ethereal HypotJiesis of Light. [Jan., 



that force, inasmuch as the imdulatory theory is now pretty generally 

 accepted, but from opposing views as to the medium upon and through 

 which it operates. It is needless for me to dwell long upon the undu- 

 latory theory, but, for the government of those who have not devoted 

 much attention to the subject, it may be as well to mention that 

 formerly light was not regarded as a force acting upon matter, but 

 was supposed to consist of particles or atoms emitted by the luminous 

 body, whilst electricity was considered an imponderable fluid which 

 travelled through the substances electrified. Now, strangely enough, 

 the views of the more advanced physical philosophers are to some 

 extent reversed. Mr. Grove considers the electrical spark, at least, 

 to consist generally of projected particles of the electrified substance,* 

 and he gives apparently satisfactory reasons for so doing, whilst 

 Professor Tyndall thinks that electricity may be a force acting upon 

 " condensed ether which surrounds the atoms" of matter.t And on 

 the other hand, as we shall presently find, the latter enthely dis- 

 cards the notion of any known substance as the vehicle of hght, 

 whilst Grrove considers it to be a force acting upon gross but highly 

 attenuated matter. But as I have already said, all are agi'eed upon 

 the dynamical theory of light, first propounded by Huyghens in 

 Newton's time, and afterwards supported and estabhshed in this 

 country by Dr. Young ; and this theory attributes to light a simi- 

 lar, though not exactly the same joroperty, as sound, regarding it 

 as a force which causes undulations of marvellous rapidity in the 

 medium through which it travels. In the case of sound, the jmssage 

 of the force is admitted on all hands to bo through known matter, 

 and it is well known that a vacuum is incapable of transmitting 

 sound ; but in that of light, which passes from sphere to sphere in 

 the universe, and traverses a vacuum with apparently greater 

 facility than air, it is obviously necessary either to discover or to 

 suppose a medium for its transmission. That there is such a 

 medium in interplanetary space is most probable, for hght occupies 

 iwie in its passage, corresponding with the distances between tlie 

 luminous bodies from which it emanates and the spheres it illumi- 

 nates, and therefore (in the case of our sun and earth for example) 

 it cannot be the atmosphere alone which oilers resistance to its 

 passage. There is most likely matter of some kind, however at- 

 tenuated, in space ; and this is shown, not alone by the impeded 

 passage of light, but by the retarded motions of the comets. 

 But what is that interplanetary matter ? 



* "Tho C'lcciiic Fpnrk, tlic bruf-li, and similar pliciii'iiniia, the (ikl theories 

 rcp;arclc(l -as iu'tiiiil ein,iiiaii(ins of the inait( r or fhiid ICleetrieity. I venture to 

 regiird flieni an pnxhiced l\v an ennsbion of tlie niatirial itself from whence they 

 isssue, and a molecular action of the gas, or intcrmc<linm, through or across wliioh 

 they are transniitt(Ml." — 'Correlation of Physical Forces and Continuity,' p. 112. 

 5th edition. liongmans : see also p. ISl. 



t 'Heat na a Mode of Motion,' p. 21 Ci, note. 'Jnd edition. Longmans. 



