29G FOURTH REPORT 1834. 



few instances, been mathematically expressed and developed ; 

 and accordingly this theory presents but rarely those points of 

 contact with experimental truth by which alone it can be judged. 



This signal difference in the present state of the two theories 

 has been by some ascribed to a difference in the intellectual 

 poM'er by which they have been worked ; and it has been said 

 that had the Newtonian theory been cultivated with the same 

 zeal and talent as the Huygenian, it might have had equal 

 triumphs to boast of. This position, I confess, appears to me 

 altogether untenable. With respect to the implied fact, it may 

 be enough to observe that Newton and Laplace were both en- 

 gaged on one side of the question ; and I believe I may add that 

 among the supporters of the wave-theory of light there are few 

 who have not had to encounter early predilections in favour of 

 the theory of emission. The nature and laws of projectile 

 movement are far more familiar to every lover of mechanical 

 philosophy than those of vibratory propagation ; and the tri- 

 umphant career of the former branch of this science, in its appli- 

 cation to the movements of the heavenly bodies, is in itself 

 sufficient to induce every one to lean to a theory which proposes 

 to account for the phenomena of light on similar principles. 

 As to the opinion itself, it seems highly improbable, to say the 

 least, that two theories so widely separated should run hand in 

 hand in their explanation of phenomena. There is indeed one 

 case, and that a striking one, of this kind : — The fundamental 

 laws of reflexioji and refraction are exact and necessary conse- 

 quences of each of these theories ; but I believe their history 

 affords no parallel instance. 



An unfruitful theory may, however, be fertilized bjr the addi- 

 tion of new hypotheses. By such subsidiary principles it may 

 be brought up to the level of experimental science, and appear 

 to meet the accumulating weight of evidence furnished by new 

 phenomena. But a theory thus overloaded does not merit the 

 name. It is a union of unconnected principles, Avhich can at 

 best be considered but as supplying the materials for a higher 

 generalization. Its very complexity furnishes a presumption 

 against its truth ; for the higher we are permitted to ascend in 

 the scale of physical induction, the more we perceive of that 

 harmonjr, and unity, and order, which must reign in the works 

 of One Supreme Author. The theory of emission, in its present 

 state, exhibits all these symptoms of unsoundness ; but there is 

 something stronger than mere presumption against it. It will 

 appear, I think, upon a fair review, that in almost every in- 

 stance in which it has been developed, its consequences are at 



