!^78 Professor Powell on the Nature qf'tfie 



throughout the planetary world ; — and that it has been found 

 continually to apply with still -increasing exactness, in propor. 

 tion to the advancing precision of observation ; — this accumulat- 

 ing species of proof is what constitutes the ground of its claim 

 to be received as the true theory of the world. 



Professor Airy has admirably enforced this view of the sub- 

 ject in his paper in the Journal of Science, June 1833, and fur- 

 ther adds, — " If at the time of inquiring into the mutual action 

 of bodies on each other, Newton had insisted on including in 

 his general theory (whatever it might be), the effects of what we 

 now call magnetism and capillary attraction, the theory of gra- 

 vitation would never have been formed. By leaving these as 

 subjects for future investigators, and by reducing to law the 

 preponderating set of phenomena, he was able to form the most 

 complete cosmical theory that has ever appeared. Many years 

 passed before those supplementary laws were reduced to a sim- 

 ple form ; yet by the consent of the world, the theory of gravita- 

 tion, though imperfect as a theory of attraction, though some- 

 times completely disguised by the forces which Newton left un- 

 explained, was adopted as a true system. That the existing 

 theory of undulations stands in the same relation to the complete 

 theory of light, as Newton's universal gravitation to the com- 

 plete theory of attraction, I have not the slightest doubt."" 



In fact, the two cases are throughout strictly parallel. We 

 shall perceive, in the speculations often pursued, — in the dis- 

 tinction to be observed between them and the legitimate theory, 

 — and in the nature of its evidence and its claims to acceptance, 

 the exact counterpart of what we have just referred to in the 

 case of gravitation. All the ordinary facts of optics indicate a 

 motion of translation or propagation in space of something, or 

 of some effect, or influence, whose rectilinear direction consti- 

 tutes a ray of light. The more obvious facts are consistent 

 with the idea simply of a motion of translation of projected par- 

 ticles. But this is not a necessary supposition ; and such a mo- 

 tion as these facts require, may equally arise from some combi- 

 nation of other motions whose resulting effect may be propa- 

 gated in rectilinear directions. When, however, we come to the 

 facts of the interferences, they absolutely require the existence 

 of some of these other kinds of action. That two rays of light. 



