1869.] The Ethereal Hypothesis of Light. 13 



conclusions on this interesting subject. There is no reason why 

 the form of matter which is behevecl to serve as the vehicle of 

 light should be so extremely attenuated as the " ether " is sup- 

 posed to be, except the necessity which seems to exist in the 

 minds of the etherealists of its permeating all other matter ; but 

 when we look at the known attenuated forms of matter, we find 

 that even the most highly rarefied are unable to penetrate certain 

 dense substances, and pass thi'ough porous ones slowly. Pro- 

 fessor Faraday found that the materials upon which he operated 

 were pervious to the passage of gases and vapours, but what must 

 be the nature of that " ether," through which the light waves are 

 supposed to speed, undergoing transformations in their passage, 

 and which must be continuous in its presence through the various 

 dense substances composing " Sonnenstein." First, it must pass 

 through a thickness of glass and through felspar, in both of which 

 it must serve as the vehicle of colourless light; then it must 

 be agitated within an embedded crystal, or if there be two super- 

 posed, then through both and the intervening felspar, and in all 

 three it must serve as the medium for the force which subsequently 

 becomes apparent to the sense as orange light ; then another layer of 

 felspar intervenes ; next, flint and crown glass ; and then it passes 

 through air, a form of matter in which the hypothetical ether may 

 be sujDposed to agitate freely. But here its course is not ended; 

 lens after lens of the microscope, each with its particles closely 

 packed, and humour after humour of the eye must all be filled with 

 this attenuated "ether," and must afford space for and be accom- 

 modated to instantaneous changes in its varied vibrations. 



I have no wish to dogmatize upon this difficult theme, my 

 purpose being, as stated at the outset, to present a few phenomena 

 for the consideration of the reader, and to suggest such inquiries as 

 seem to me calculated to throw light on the subject. From the 

 foregoing remarks, however, it will be clear that I lean to Grove's 

 view of the purely material character of the substances which serve 

 as the vehicles of light, and that, notwithstanding the need which 

 appears to exist for some special medium, either elementary or 

 compound, to provide for its passage across a " vacuum," yet I 

 cannot admit either the possibility or necessity for a specific " ether " 

 which permeates all matter. For although the chain of hypotheses 

 which must be employed to supj^ort the one hypothesis of a homo- 

 geneous sj)ecific " ether," filling all space and " fitted mechanically 

 for the transmission of the vibrations of light and heat,"* and per ■ 

 meating all kinds of gross matter, may seem necessary and justifiable 

 in the minds of those who are more accustomed than I am to con- 

 sider these phenomena, yet it seems to me that before the hypo- 



* Tyndall's ' Eadiution,' p. 8. 



