of Light ; and a new Theory of Fision. 427 



duced in proof of the existence of any thing, by analogy, neces- 

 sitv, and matter of fact ; it accounts for all the phaenomena of 

 light passing through a medium, without involving any difficulty, 

 which cannot be said of any other system ; and if further proof 

 should still be required, it is to be found in the utter incapacity 

 of the old hypothesis to produce the phaenomena it is intended 

 to explain. 



" What kind of action or disposition this is," says Sir Isaac 

 Newton in speaking of his hypothesis of alternate reflection and 

 refraction, " whether it consists of a circulating or a vibrating 

 motion of the rav, or of the medium, or something else, I do 

 not here inquire*. Those who are averse from assenting to any 

 new discoveries but such as they can explain by an hypothesis, 

 may for the present suppose, that as stones by falling upon water 

 put the water into an undulating motion, and all bodies by per- 

 cussion excite vibrations in the air, so the rays of light by im- 

 pinging on any refracting or reflecting surface excite vibrations 

 in the refracting or reflecting medium or substance, and by ex- 

 citing them agitate the solid parts of the refracting or reflecting 

 body, and by agitating them cause the body to grow warm or 

 hotf 5 that the vibrations thus excited are propagated in the re- 

 fracting or reflecting medium or substance, much after the man- 

 ner that vibrations are propagated in the air for causing sound, 

 and move faster than the rays, so as to overtake them ; and that 

 when any ray is in that part of the vibration which conspires 

 with its motion, it easily breaks through a refracting surface, but 

 (vhen it is in the contrary part of the vibration which impedes 

 its motion, it is easily reflected ; and, by consequence, that every 

 rav is successively disposed to be easily reflected, or easily trans- 

 mitted, by every vibration which overtakes it. But whether this 

 hypothesis be true or false, I do not here consider. I content 

 myself with the bare discovery that the rays of light are by some 

 cause or other alternately disposed to be reflected or refracted for 

 many vicissitudes." 



Now it must be evident to every one who reads this paragraph 

 with attention, that no part of the argument it contains is 



• This reasoning, to say tlie least of it, is vague and unsatisfactory, and 

 i-an hardly be said to lu-count for the pliitnomena, when it is uncertain what 

 the action is, whether it be a circulating or a vibrating motion, and whether 

 tliis motion be in the ray, the medium, or any thing else. 



t If the solid parrs of a glass lens grew hot in consequence of light pars- 

 ing through, would it not be perceptible to the touch ? and after all, what 

 effect can the heated parls of a medium have upon the sun s rays, which are 

 infinitely hotter than they can possibly be? Even if they have any oflect, 

 they must alternately grow hot and cold as many times in a moment of time 

 n.* there are rays of light reflected and refracted in ttie medium in that time ; 

 whivii is ub^ulutcly impottsible. 



Q (J 2 grounded 



