ON THE THEORY OF LIGHT AND COLOURS. 



6h 



▼eyed through the vacuum by the vibrations 

 of a much subtiler medium than air ? — And 

 is not this nT^dium the same with that me- 

 dium by which light is refracted and re- 

 flected, and by whose vibrations hght com- 

 municates heat to bodies, and is put into fits 

 of easy reflection, and easy transmission ? 

 And do not the vibrations of this medium in 

 hot bodies, contribute to the intenseness 

 and duration of their heat ? And do not hot 

 bodies communicate their heat to contiguous 

 cold ones, by the vibrations of this medium 

 propagated from them into the cold ones ? 

 Aiid is not this medium exceedingly more 

 rare and subtile than the air, and exceed- 

 ingly more elastic and active? Aijd doth it 

 not readily pervade all bodies ? And is it 

 not by its elastic iorce, expanded through all 

 the heavensf — May not planets and comets, 

 and all gross bodies, perform their motions 

 in this ethereal medium? — And may not its 

 resistance be so small, as to be inconsider- 

 able? For instance, if this ether (for so I 

 will call It) should be supposed 700 000 times 

 more elastic than our air, and above 700 000 

 times more rare, its resistance would be 

 about tiOOOOOOOO times less than that of wa- 

 ter. And so small a resistance would scarce 

 make any sensible alteration in the motions 

 of the planets, in ten thousand years. If 

 any one would ask how a medium can be so 

 rare, let him tell me — how an electric body 

 can by fricfticm emit an exhalation so rare 

 and subtile, ami yet so potent ? — And how the 

 efHuvia of a magnet can pass through a 

 plate of glass, without resistance, and yet 

 turn a magnetic needle beyond the glass r" 

 (Optics, Qu. 18, 22.) 



Hypothesis ii. Undulations are excited 

 in this Ether whenever a Body beeomes lumi- , 

 nous. 



Scholium. I use the word undulation, in 

 preference to vibration, because vibration is 

 generally understood as implying a motion 

 which is continued alternately backwards and 

 forwards, by a combination of the momen- 

 tum of the body with an accelerating force, 

 and which is naturally more or less perma- 

 nent ; but an undulation is supposed to con. 

 sist in a vibratory motion, transmitted succes- 

 sively through different parts of a medium, 

 without any tendency in each particle to 

 continue its motion, except in consequence 

 of the transmission of succeeding undula- 

 tions, from a distinct vibrating body ; as, ia 

 the air, the vibrations of a chord produce the 

 undulations constituting sound. 



PASSAGES FROM NEWTON. 



" Were I to assume an liypothesis, it 

 should be this, if propounded more generally 

 so as not to determine what light is, further, 

 than that it is something or other capable of 

 exciting vibrations in the ether; for thus it will 

 become so general, and comprehensive of 

 other hypotheses, as to leave little room for 

 new ones to be invented." (Birch, III. 249> 

 Dec. 1675.) 



" In the second place, it is to be supposed, 

 that the ether is a vibrating medium, like air, 

 only the vibrations far more swift and mi- 

 nute ; those of air, made by a man's ordinary 

 voice, succeeding one another at more than 

 half a foot, or a foot distance; but those of 

 ether at a less distance than the hundred 

 thousandth part of an inch. And, as in ain 

 the vibrations are some larger than others' 

 but yet all equally swift, (for in a ring of bells 

 the sound of every tone is heard at two or 

 three miles distance, in the same order that 

 the bells are struck,) so, I suppose, the ethe- 

 real vibrations difter in bigness, but not in 



