Evidence in support of a Theory of' Light. S77 



the palpable and apparently insurmountable difficulty of the 

 unequal refrangibility ol' light, have been all urged as amounting 

 to positive refutations of the theory, and as not only Jointly, but 

 even singly, Jhtul to its claims, and calling for its entire rejection. 



Without here pretending to enter upon the several points of 

 objection in detail, our object will be rather to shew, by a brief 

 reference to the real nature of the evidence, that the arguments 

 on both sides have been overstated — that the claims of the theory 

 have been as unduly urged on the one hand as disparaged on 

 the other, and in either case owing principally to a misconcep- 

 tion of the nature of those claims. We may fully admit, on 

 the one hand, that the proofs are far from being demonstrative, 

 and on the other we may allow the deficiencies to be as nume- 

 rous, and the objections as strong, as they are represented ; and 

 yet, notwithstanding, the theory, when understood in its correct 

 sense, may stand on its proper ground as firmly as ever. 



Its ground of evidence has been compaied with that of the 

 system of gravitation. In this system many, doubtless, have 

 speculated on the physical cause of attraction ; but the true phi- 

 losopher, if he ever indulge in such speculations, keeps them 

 carefully distinct from the real investigations of the legitimate 

 theory. Many hypotheses may have been advanced, on the one 

 hand, as to the actual nature of that force which pervades the 

 system of the world ; and on the other, the existence of an at- 

 tracting influence, or real physical connexion, reciprocally uniting 

 the minutest particles of matter in the remotest regions of the 

 universe, has been censured as visionary and absurd ; but the 

 credit of the real theory is in no way involved in these specula- 

 tions. The real philosopher recognises gravitation only as a 

 name for the general Jact of a tendency in matter to approach 

 directly as the mass, and inversely as the square of the distance. 

 The question is. Whether the subsistence of this law, which we 

 know by direct observation in regard to the earth and bodies 

 near it, as a " vera causa,'''' will, by the deduction of its various 

 consequences, suffice to explain, and enable us to calculate, all 

 the motions of the remote bodies of our system ? That it has 

 been found to do so, not only in the greater and more notable 

 cases, but even in all the most minute and nearly inappreciable 

 results of the vast complication of impulses which arc in action 



