relative to Black, Watt, and Cavendish. 131 



his filaments of air, or his aethereal globules interposed between 

 them, with attractive or repulsive powers. 



The genius of Hook, so comprehensive of clear physical 

 notions, soon lent to this luminiferous aether the mechanical 

 attribute which it needed, and added the notion of vibratory 

 pulses, — a notion which was instantly reduced by Newton to 

 the form most competent to account for the phenomena*, 

 and on which Huygens founded, and Young with his illus- 

 trious coadjutors have gone far to finish, the mathematical 

 fabric of the undulatory theory of light, as it is now commonly 

 received. 



So necessary indeed to any account of the phenomena of 

 light and colours did the admission of such a medium appear, 

 that Wallis, who not only rejected the use which Huygens and 

 others proposed to make of it in explaining the extraordinary 

 height at which mercury, purged of air, may be suspended in 

 a tube, but denied it the properties of elasticity and weight, 

 nevertheless did not scruple to say, " That there is in our air 

 a body more subtle than the fumes and vapours mixed with it 

 in our lower region is very certain : but whether that subtle 

 body be, as Dr. Garden seems to suppose, much heavier than 

 our common air, I much doubt, and rather think it is not, not 

 having hitherto had any cogent experiment either to prove it 

 heavy or elastic ; but it may, for aught I know, be void as 

 well of weight as spring, and what is found of either in our 

 common air may be attributed to the other mixtures in itf." 



le mouvement des corps lumineux ne peut passer jusques a nos yeux, qu'il 

 n'y passe quelque chose de materiel qui sorte de ces corps ; car je ne fais en 

 ces deux pages qu'expliquer la comparaison d'un aveugle, laquelle j'ai prin- 

 cipalement apportee pour faire voir en quelle sorte le mouvement peut passer 

 sans le mobile ; et je ne crois pas que vous pensiez lorsque cet aveugle touche 

 son chien de son baton qu'il faille que ce chien passe tons le long de son 

 baton jusque a sa main, afin qu'il en sent les mouvements. Mais afin que 

 je vous reponds in forma, quand vous dites que le mouvement n'est jamais 

 sans le mobile, distinguo; car il ne peut veritablement etre sans quelque 

 corps, mais il peut bien etretransmis d'un corps en un autre, et ainsi passer 

 des corps lumineux vers nos yeux par l'entremise d'un tiers, a savoir, comme 

 je dis en la page 4, par l'entremise de l'air et des autres corps transparents, 

 on comme j'explique plus distinctement en la page 6, par l'entremise d'une 

 maticre fort subtile qui remplit les pores de ces corps et s'etend depuis les 

 astres jusques a nous" (p. 240), 



* Phil. Trans., No. 88, p. 5088. An. 1672. "The most free and natural 

 application of this hypothesis I take to be this: That the agitated parts of 

 bodies, according to their several sizes, figures, and motions, do excite vi- 

 brations in the aether " &c. 



t Phil. Trans. No. 171, p. 1002. 



[To be continued.] 



