10 The Ethereal Hypothesis of Light. [Jan., 



and of its resolution as above stated. And now let us inquire 

 whether and in what manner hght differs in its operation li-om 

 the other known forces, or " modes of motion." First, as regards 

 its peuetrahihty. If we do not assume that the medium which 

 serves as its vehicle until it reaches a solid object enters into the 

 constitution of that object, but that it is arrested at its surface, 

 then we shall find that the impact produces the same efiects in 

 the case of hght as in other forces, and, moreover, we shall avoid a 

 grave difficulty attending the assumption that the hypothetical 

 " ether " serves as the vehicle of hght in and through the sohd 

 body, namely, that porous substances through which the " ether " 

 should pass most freely are opaque, whilst dense forms of the iden- 

 tical substance are transparent. 



If we place a number of billiard-balls in a row thus (Fig. 1), 



Fir,. 1. 



and drive a ball or other object against the terminal baU A, the 

 force traverses the whole closely-packed series instantaneously, and 

 the baU Z starts forward at once ; * but if the balls are disposed 

 as at Fig. 2, and A be struck with the same force, Z -will only move 

 slightly forward, and not so instantaneously as before ; and if they 

 be disposed as at Fig. 3, not touching each other, then if A be 

 struck, Z will not move at all, but some of the intervening balls will 

 fly off laterally at different angles. Now the fii'st position (Fig. 1) 

 may be assumed to be that of the particles of matter in a dense, 

 and the other two in porous bodies, and if we regard light as a 

 " mode of motion," there is nothing abnormal in its passing more 

 rapidly through, or, to speak correctly, in its traversing the particles 

 of a dense than a porous body, merely on account of the density, 

 and provided the particles be conveniently disposed. Again, air 

 is the medium of sound, as ether is supposed to be that of light ; 

 but when sound, or rather the agitated air,t impinges upon the 



* Professor Tyiidall employs tliis illustration to exhibit the effect of sound. 



t Perhiij)s it would be more correct to say Hint sound is the vibration of tlic 

 sensible, as heat and light are the vibrations of tlic insensible jiarls of an object. 

 The latter definition is Locke's. 



