534< The Astronomer Royal's Remarks 



change of velocity of the undulations, and a power at the same 

 time of changing the direction of progress in a degree exactly 

 corresponding to the change of velocity : these changes are 

 in the simplest and most natural manner possible explained 

 by the theory of true waves in which the swell produced by 

 every particle is propagated in all directions through a very 

 large angle, while (as I apprehend) it will be found somewhat 

 difficult to modify a theory of radial shakes so as to explain 

 them ; therefore I conceive it demonstrated that the propaga- 

 tion of true waves takes place through the air to its utmost 

 borders. Beyond the existence of sensible air we can make 

 no experiments ; and I am free to concede that if we supposed 

 the air and its accompanying aether [if different] to terminate 

 at a distinct frontier, and if we supposed the transversal shakes 

 to be propagated radially through the planetary spaces to that 

 frontier, and then supposed each shake, as it presented itself, 

 to be the origin of a spreading swell through the asther, the 

 phaenomena of light would be explained. But here a re- 

 markable circumstance forces itself on our minds. A mo- 

 ment's consideration will show that at this frontier the course 

 of the light will be subject to refraction, in just the same way 

 as if the incident light had consisted of waves, and following 

 the same law as depending on the velocity of propagation. 

 Now it is abundantly established that at the boundary of our 

 air there is no sensible refraction, that is, that the velocity of 

 the propagation is not sensibly altered. Now is it not a very 

 curious circumstance that there should be a system of radial 

 shakes outside and a system of true waves inside which pro- 

 pagate the undulations with exactly the same velocity? Is 

 there any philosopher who would be inclined to receive as 

 true this suggestion of two independent causes of velocity, and 

 tliis exact adjustment of independent velocities, when the ad- 

 justment will necessarily exist if the same vibrating medium or 

 aether occupies all space? Not I, certainly. However well- 

 disposed I might be to admit any such saltus of nature at the 

 surface of glass or crystal where the phaenomena of light are 

 totally changed, I cannot bring myself to believe in it as ex- 

 isting either through the air where the change of phenomena 

 is gradual, or at the limits of the air where there is no change 

 at all. In a word, I must have the same theory of light for 

 the planetary spaces as for the air in which our experiments 

 on diffraction are made; and that theory must be the theory 

 of true waves. 



I do not insist on the novelty of the conception, that lateral 

 influences take place in a travelling succession along a radial 

 line, in a manner different from anything whatever that we 



