ON LIGHT. 273 



i.e., at right angles to the surface. Its velocity esti- 

 mated in this direction will therefore be greater within 

 the medium than without while that parallel to the 

 surface remains unchanged : the force in that direction 

 being nil. The direction of the motion therefore will 

 be more highly inclined to the surface within the medium 

 than without, in the same manner and for the very same 

 reason, that the path of a projectile shot obliquely down- 

 wards from the top of a hill makes a greater angle with 

 the horizon when it reaches the ground than it did in the 

 commencement of its descent. And the conclusion, on 

 strict dynamical principles, is the same in both cases. 

 Supposing the initial velocity of projection the same, the 

 sines of the angles made by the direction of the motion 

 with the vertical or perpendicular to the surface, at the 

 beginning, and at the end of the descent (i.e., in the case 

 of light, those of the angles of incidence and refraction), 

 will be to each other in an invariable proportion, the 

 total height of the descent being the same. Thus we see 

 that the law of refraction is satisfactorily accounted for, 

 on the corpuscular hypothesis j and that, on that theory, 

 the velocity is greater in the interior of a refracting 

 medium than in empty space ; and the more so, the 

 greater the refractive power. 



(57.) Let us now see in what sort of conclusion we are 

 landed as to the intensity of the forces we have pressed 

 into our service. To consider only the reflective force, 

 we have this to guide us that, supposing the incidence 

 perpendicular, and the light therefore reflected back by 

 the path of its arrival, that force must have been suffici- 



s 



