56 On the MOTION of LIGHT. 



motions of which we are entirely ignorant. For this reafon, 

 we cannot fay what is the abj'olute motion of light, nor afcertain 

 from faSi what changes it undergoes in its obferved refradions 

 and refledions. Since, therefore, we cannot fimpUfy the phe- 

 nomena which nature prefents to our view, we cannot eftabUfh 

 thofe general laws which would be the foundation of a phyfical 

 theory. 



The only other method which feems to remain, is to take 

 up fuch opinions concerning the nature of light, as feem moft 

 rationally deducible from the phenomena which we obferve, 

 and then to deduce, by the eftablillied principles of mechanics, 

 fuch confequences as fhould arife from the adlion of refrad:ing 

 and refleding fubftances upon this hypothetical light. We 

 fhould then feledt fuch of thefe confequences as will admit of 

 a comparifon with obfervation. If thefe confequences ihall be 

 found inconfiftent with obfervation, the hypothefis concerning 

 the nature of light mufl: be rejected, and trial muft be made of 

 a new one. But if they fhould be found to agree with obferva- 

 tion, and at the fame time be fufficiently various, we may 

 then admit the hypothefis to have a degree of probability pro- 

 portioned to the extent of the comparifon which we have made 

 of its confequences with obfervation ; we may then difcover 

 by this means parts of a hypothefis which muft be admitted as 

 true, although the hypothefis cannot be demonftrated in its full 

 extent. 



I AM acquainted with two hypothefis only concerning the 

 mechanical nature of light, which, in the opinions of the 

 learned, feem rationally deduced from the phenomena. The 

 firft is that which is advanced by Sir Isaac Newton, in feve- 

 ral parts of his celebrated writings. He fays that light may 

 perhaps confifl of fmall particles emitted by the Ihining body 

 with prodigious velocity, which are afterwards adled upon by 

 other bodies, with attracting or repelling forces like gravity, 

 vjhich defleft them from their redilineal courfes in refradions 



and 



