Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 1.djvu/409

Rh which fall upon the knife are firt inflected in the air before they touch the knife. And the cae is the ame of the rays falling upon glas. The refraction therefore is made, not in the point of incidence, but gradually, by a continual inflection of the rays; which is done partly in the air before they touch the glas, partly (if I mitake not) within the glas, after they have entred it as is repreented (Pl. 25. Fig. 7.) in the rays ckzc, buyb, ahxa, falling upon r, q, p and inflected between k and z, i and y, h and x. Therfore hecaue of the analogy there is between the propagation of the rays of light, and the motion of bodies, I thought it not amis to add the following propoitions for optical ues; not at all conidering the nature of the rays of light, or enquiring whether they are bodies or not; but only determining the trajectories of bodies which are extremely like the trarectories of the rays.

Supoing the ine of incidence upon any uperficies to be in a given ratio to the ine of emergence; and that the inflection of the pas of thoe bodies near that uperficies is performed in a very hort pace which may be confirmed as a point, it is required to determine uch a uperficies as may caue all the corpucles iuing from any one given place to converge to another given place.

Let A (Pl. 25. Fig. 8.) be the place from whence the corpucles diverge; B the place to