9 6 On the MOTION of LIGHT. 



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

 we cannot fay what is the abfolute motion of light, nor afcertain 

 from facl what changes it undergoes in its obferved refractions 

 and reflections. Since, therefore, we cannot Amplify the phe- 

 nomena which nature prefents to our view, we cannot eftablifh 

 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 mod 

 rationally deducible from the phasnomena which we obferve, 

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

 fuch confequences as fhould arife from the action of refracting 

 and reflecting fubftances upon this hypothetical light. We 

 fhould then felect fuch of thefe confequences as will admit of 

 a comparifon with obfervation. If thefe confequences (hall be 

 found inconfiftent with obfervation, the hypothecs concerning 

 the nature of light mud be rejected, and trial mult 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 mining body 

 with prodigious velocity, which are afterwards acted upon by 

 other bodies, with attracting or repelling forces like gravity, 

 which deflect them from their rectilineal courfes in refractions 



and 



