20 REPORT ON PHYSICAL OPTICS. 



a point almost, if not entirely, as advanced as that to which the 

 theory of universal gravitation was pushed by the single-handed 

 efforts of Newton. Varied and comprehensive classes of phenomena 

 have been embraced in its deductions ; and where its progress has 

 been arrested, it has been owing in a great degree to the imper- 

 fections of that intricate branch of analysis by which it was to 

 be unfolded. The principles of the theory of emission, on the 

 other hand, have in comparatively few instances been mathemati- 

 cally expressed and developed ; and accordingly this theory pre- 

 sents 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 power 

 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 un- 

 tenable. With respect to the implied fact, it may be enough to 

 observe that Newton and Laplace were both engaged on one side 

 of the question ; and I believe I may add that among the sup- 

 porters 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 triumphant career of the former branch of 

 this science, in its application to the movements of the heavenly 

 bodies, is in itself sufficient to induce everyone 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 

 reflexion and refraction are exact and necessary consequences of 

 each of these theories; but I believe their history affords no 

 parallel instance. 



An unfruitful theory may, however, be fertilized by 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 phe- 

 nomena. But a theory thus overloaded does not merit the name. 



