ON LIGHT. 28l 



in free space. In other words, this amounts to suppos- 

 ing the elastic force of the ether either to be enfeebled 

 in the interior of material bodies, or else that the move- 

 ments of its particles are in some way or otJier clogged or 

 burthened by some sort of connexion with or adhesion 

 to the material molecules among which they are dissem- 

 inated, and that more for the more refrangible rays than 

 for the less so. 



(64.) Until lately this difference of velocity between 

 the differently refrangible rays had always been consi- 

 dered an insuperable obstacle to the admission of the 

 undulatory hypothesis. All sounds of whatever pitch (it 

 was contended) travel equally fast in one and the same 

 elastic medium. The profounder researches of later 

 mathematicians, however, have shown that this conclu- 

 sion is not absolute, and that on certain suppositions 

 which are not altogether inadmissible in respect of the 

 vibrations of light, the difference is not contradictory to 

 strict dynamical laws. 



(65.) As we have attempted to form an estimate of the 

 intensity of the forces required to account for observed 

 facts on the corpuscular hypothesis, let us now attempt 

 a parallel estimate on the undulatory. And here the 

 way is equally open and obvious. Starting with the ob- 

 served facts, that sound travels in air at the rate of 1090 

 feet per second, while light is propagated through the 

 ether 186,000 miles in the same time (that is to say, 

 901,000 times as fast), we are enabled to say how many 

 fold the elastic force of the air, or its resistance to com- 

 pression, would require to be increased in proportion to 



