ASTRONOMY, ETC., OF SEVENTEENTH CENT. 227 



dium, or (zther, pervades all space and all bodies, and that within the 

 denser transparent bodies this exists in a state of greater condensation. 

 Waves, pulsations, or undulations are by some action of luminous 

 bodies set up in this medium, and are propagated in it in all direc- 

 tions, and these pulsations reaching the eye, produce the sensation 

 of sight. Under ordinary circumstances, the undulations spread from 

 the point of origin in a regular spherical form, just as the wave pro- 

 duced by dropping a stone into still water spreads along on the sur- 

 face in circles. This theory was applied by Huyghens to the expla- 

 nation of all the ordinary phenomena of reflection and refraction. 

 In reflection, the waves are thrown back from the reflecting surface in 

 a way which is easily understood by any one who has observed the 

 waves in a still pool when they arrive at an obstacle, such as' a wall. 



FIG. in. 



The undulatory theory explained very happily the laws of refraction ; 

 for as the light, according to this theory, is propagated with less velocity 

 in the denser medium when a wave arrives obliquely at the common 

 surface in passing from a rarer into a denser medium, its front will 

 be changed, and it will move in a direction nearer to the perpendicu- 

 lar. Thus, if A B, Fig. in, be the front of a wave advancing in 

 the direction of the arrow, the parts of the wave successively enter- 

 ing the denser medium will move therein with a diminished velocity ; 

 and that this change of velocity must produce a change of direc- 

 tion, will be plain if we consider the matter with a little attention. 

 Thus, while the part of the wave B is advancing to B' in the rarer 

 medium, A must have moved in the denser medium to some point 

 which is less distant from A than B' is from B. Suppose that the undula- 

 tion advances in the denser medium with three-fourths of the velocity 

 it has in the rarer, then if we describe from the centre A a circle, efg, 

 with a radius = three-fourths of B B', we are sure that* when B arrives at 



15 2 



