especl'mg the Nature of Light. 19/ 



many spiral turns, but experiments prove directly the contrary 

 with respect to light. It may be said that it is for want of the 

 tubes having a due polish : but we know that straight metallic 

 tubes which are highly polished reflect light in great quantity, 

 and with great ease, but cannot reflect it so as to cause it to 

 pass through them when they are curved. If light consists of a 

 series of undulations, it is difficult to conceive how we could 

 perceive objects clearly and distinctly : from what we observe 

 with respect to bodies thrown into a state of vibration, as when 

 a musical chord is struck, it almost apjiears impossible that the- 

 edges of bodies should be distinctly seen. Again, if light de- 

 pended on the undulations of this ethereal medium, caused by 

 the vibrations of the luminous body, as stated by Dr. Young 

 when he accounts for the phsuomena of solar phosphori — why 

 does not light always accompany sound ? For the sounding body 

 not merely imparts its vibrations to the air, but also to fluids 

 and the most solid bodies. 



It is therefore difficult to conceive that an elastic subtile fluid 

 should not also partake of this vibratory motion ; particularly as 

 it is a fluid capable of being affected by a verv slight cause so 

 as to produce light, as when light is perceived by means of 

 many pyrophori ; yet the loudest tones are not accompanied 

 with light. And on the contrary, the most intense light has not 

 the least effect on the ear, although this beautiful organ is truly 

 formed to convey the slightest vibrations to the sensorium. 

 When we contemplate the mechanism of the ear, we find an 

 organ formed so as to convev anv vibratory motion ; but the eve 

 is a piece of mechanism entirely different, and appears but badly 

 calculated to jiropagate tremors. These observations appear 

 to be in direct opposition to what might be expected if light 

 consisted in the undulation or vibration of a higlily elastic me- 

 dium. In short, the very existence of such a niediimi is entirely 

 gratuitous. The same objections also hold good against the 

 opinion advanced by Malbranche ; wiio supposed tliat there 

 was an analogy betueen sound and light, and that bodies ap- 

 peared more or less luminous according to the vibration? of a 

 eubtile matter between the eve and the object, which matter was 

 put into motion by the rapidity with which the particles of the 

 body moved. 



In tlie latter part of the lecture Dr. Young accuses Sir Isaac 

 Newton of want of candour towards Iluvgens, wlncli we may 

 here notice, in endeavouring to account for the singular refrac- 

 tion of the Iceland crystal. There are several passages in the 

 writings of this great man, which will show that the Doctor has 

 fallen into an error on this subject. Newton expressly states in 

 his 2blh i^ucry, at the end of his Treatise on OpticSj *' That to 

 N 3 explai^ 



