respecting Sound and Light. 127 



to be a million times greater than it is. For explaining the 

 phenomena of partial and total reflection, refraction, and inflec- 

 tion, nothing more is necessary than to suppose all refracting 

 media to retain, by their attraction, a greater or less quantity 

 of the luminous ether, so as to make its density greater than 

 that which it possesses in a vacuum, without increasing its elasti- 

 city ; and that light is a propagation of an impulse communi- 

 cated to this ether by luminous bodies : whether this impulse is 

 produced by a partial emanation of the ether, or by vibrations 

 of the particles of the body, and whether these vibrations are, 

 as Euler supposed, of various and irregular magnitudes, or 

 whether they are uniform, and comparatively large, remains to 

 be hereafter determined. Now, as the direction of an impulse 

 transmitted through a fluid, depends on that of the particles in 

 synchronous motion, to which it is always perpendicular, what- 

 ever alters the direction of the pulse, will inflect the ray of 

 light. If a smaller elastic body strike against a larger one, it is 

 well known that the smaller is reflected more or less powerfully, 

 according to the difference of their magnitudes : thus, there is 

 always a reflection when the rays of light pass from a rarer to 

 a denser stratum of ether; and frequently an echo when a sound 

 strikes against a cloud. A greater body striking a smaller one, 

 propels it, without losing all its motion : thus, the particles of a 

 denser stratum of ether, do not impart the whole of their motion 

 to a rarer, but, in their effort to proceed, they are recalled by the 

 attraction of the refracting substance with equal force; and thus 

 a reflection is always secondarity produced, when the rays of 

 light pass from a denser to a rarer stratum. Let x\B, Plate V. 

 Fig. 29, be a ray of light falling on the reflecting surface FG; 

 cd the direction of the vibration, pulse, impression, or conden- 



