196 Olservations upon the different Hypotheses 



This great philosopher, no less celebrated and revered for the 

 superior powers of his n)ind than for the modesty with which 

 he gave his opinion on any subject, supposed liglit not to be 

 a fluid per se, but that it was projected from the luminous body 

 with an immense velocity in all directions, consisting of a great 

 number of very small particles, and always proceeding in right 

 lines, until they were turned from that path by some particular 

 cause. Several objections have iieen brought against this opi- 

 nion of Newton's, as will be presently noticed. But in the first 

 place let us turn oiu- attention to the Huygenian theory, or more 

 properly its modification by Dr. Young in his Bakerian Lecture 

 for 1802. The Doctor observes, 1st, That a luminiferous ether 

 pervades the universe, rare and elastic in a high degree. 2dly, 

 Undulations are excited in this ether whenever a body becomes 

 luminous. 3dly, The sensation of different colours depends 

 on the frequency of vibrations excited by light in the retina. 

 4thly, All material bodies have an attraction for the ethereal me- 

 dium, by which it is accumulated within their substance, and for 

 a small distance around them, in a state of greater density, and 

 not of greater elasticity. Dr. Young first quotes several pas- 

 sages from Newton's works, to prove that Newton himself had 

 recourse to an extremely elastic and subtile medium, which re- 

 flects and refracts light by fits of easy transmission ; that this 

 medium is far more elastic, rare, and active than air. He then 

 proceeds to prove that the undulations of this elastic medium are 

 light ; and in his 9th prop, he states, that radiant light con- 

 sists in undulations of the luminous ether. 



To this hypothesis of Dr. Young's there appear to be many 

 objections, and some of them, I am inclined to think, almost in- 

 surmountable. Newton has stated very clearly, " that a pressure 

 on a fluid medium, i. e. a motion propagated by such a medium 

 beyond any obstacle which impedes any part of its motion, can- 

 not be propagated in right lines, but will be always inflecting 

 itself every way to the quiescent medium beyond that obstacle." 

 He further adds, " The waves, pulses or vibrations of the air, 

 wherein sound consists, are manifestly inflected, though not so 

 considerably as the waves of water ; and sounds are propagated 

 with equal ease throuqh crooked tubes as through straight lines; 

 but liglit was never known to move in any curve, nor to inflect 

 itself ad nmham." In reply to this objection Dr. Young ob- 

 serves, *' Sounds are propagated through crooked passages, be- 

 cause their sides are capable of reflecting sound, just as light 

 would 1)6 propagated through a l)ent tube if perfectly polished." 

 The statement in the latter part of this passage can carry but 

 little weight with it, as it is merely an assumption. Sound, it 

 is well known, passes with ease through a tube consisting of 



many 



