xxiii.] THE USE OF HYPOTHESIS. 515 



space ; it is .in positive contradiction to the old dictum 

 that nothing can act but tlirough some medium. It is 

 even more puzzling that the force acts in perfect indiffer- 

 ence to intervening obstacles. Light in spite of its 

 extreme velocity shows much respect to matter, for it is 

 almost instantaneously stopped by opaque substances, and 

 to a considerable extent absorbed and deflected by trans- 

 parent ones. But to gravity all media are, as it were, 

 absolutely transparent, nay non-existent ; and two particles 

 at opposite points of the earth affect each other exactly as 

 if the globe were not between. The action is, so far as 

 we can observe, instantaneous, so that every particle of the 

 universe is at every moment in separate cognisance, as it 

 were, of the relative position of every other particle through- 

 out the universe at tliat same moment of time. Compared 

 with such incomprehensible conditions, the theory of 

 vortices deals with commonplace realities. Newton's 

 cidebrated saying hypotheses non Jingo, bears the appearance 

 of irony; and it was not without apparent grounds that 

 Leibnitz and the continental philo60])liers charged Newton 

 with re-introducing occult powers and qua,lities. 



The undulatory theory of light presents almost equal 

 difficulties of conception. We are asked by physical 

 philosophers to give up our prepossessions, and to believe 

 that interstellar space which seems empty is not empty at 

 all, but filled with something immensely more solid and 

 elastic than steel. As Young himself remarked.^ " the 

 luminiferous ether, pervading all space, and penetrating 

 almost all substances, is not only highly elastic, but 

 absolutely solid ! ! ! " Herschel calculated the force which 

 may be supposed, according to the undulatory theory of 

 light, to be constantly exerted at each point in space, and 

 finds it to be 1,148,000,000,000 times the elastic force of 

 ordinary air at the earth's surface, so that tlie pressure 

 of ether per square incli nmst be about seventeen billions 

 of pounds.^ Yet we live and move without appreciable 

 resistance through this medium, immensely harder and 

 more elastic than adamant. All our ordinary notions 

 nmst be laid aside in contemplating such an hypothesis; 



' Vounj;,''s Wiirlis. vol. i. p. 415. 



' Familiar Luctures on iicienlijic Subjects, p. 282. 



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