' 



THEORY OF LIGHT AND COLOURS. gX 



" ones ? And is not this medium exceedingly more rare and 

 " fubtile than the air, and exceedingly more elaflic and aftiver 

 a And doth it not readily pervade all bodies ? And is it not, 

 ,f by its elaflic force, expanded through all the heavens ? — 

 u May not planets and comets, and all grofs bodies, perform 

 " their motions in this ethereal medium ? And may not its re- 

 u fiftance be fo fmall, as to be inconliderable ? For inftance, if 

 " this ether (for fo I will call it) fhould be fuppofed 700,000 

 u times more elaflic than our air, and above 700,000 times 

 " more rare, its refinance would be about 600,000,000 lefs 

 " than that of water. And fo fmall a refiftance would fcarce 

 " make any fenfible alteration in the motions of the planets, in 

 " ten thoufand years. If any one would afk how a medium 

 ** can be fo rare, let him tell me, How an eleclric body can by 

 ** friction emit an exhalation fo rare and fubtile, and yet fo 

 " potent ? And how the effluvia of a magnet can pafs through 

 " a plate of glafs without refiftance, and yet turn a magnetic 

 ** needle beyond the glafs ?" (Optics, Qu. 18, 22.) 



HYPOTHESIS II. 



Undulations are excited in this Ether tvhenever a body becomes 

 luminous. 



Scholium. I ufe the word undulation, in preference to vi- Hypoth. II. 

 bration, becaufe vibration is generally underftood as implying a ?* 5 6 ht c °n fifts 

 motion which is continued alternately backwards and forwards, dulationT "^ 

 by a combination of the momentum of the body with an acce-s 

 lerating force, and which is naturally more or lefs permanent ; 

 but an undulation is fuppofed to confift in a vibratory motion, 

 tranfmitted fucceflively through different parts of a medium, 

 without any tendency in each particle to continue its motion, 

 except in confequence of the tranfmiffion of fucceeding undu- 

 lations, from a diftinct vibrating body ; as, in the air, the vi- 

 brations of a chord produce the undulations conftituting 

 found. 



PaJJages from Newton, 



" Were I to affume an hypothefis, it mould be this, if pro- Detail of the 

 " pounded more generally, fo as not to determine what light hypoth. by , 

 " is, further than that it is fomething or other capable of ex _ Newton * Li S ht « 

 " citing vibrations in the ether; for thus it will become fo ge- 

 c * neral and comprehenfive of Other hypothefes, as to leave 

 " little room for new ones to be invented/' (Birch. Vol rf III. 

 p. 249. Dec. 1675.) 

 Vol II.— June, 1802. G « in 



