which fall upon the knife are firſt inflected in the air before they touch the knife. And the caſe is the ſame of the rays falling upon glaſs. 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 glaſs, partly (if I miſtake not) within the glaſs, after they have entred it
as is repreſented (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 hecauſe of the analogy there is between the propagation of the rays of light, and the motion of bodies, I thought it not amiſs to add the following propoſitions for optical uſes; not at all conſidering 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.
Proposition XCVII. Problem XLVII.
Supoſſing the ſine of incidence upon any ſuperficies to be in a given ratio to the ſine of emergence; and that the inflection of the paſſs of thoſe bodies near that ſuperficies is performed in a very ſhort ſpace which may be confirmed as a point, it is required to determine ſuch a ſuperficies as may cauſe all the corpuſcles iſſuing from any one given place to converge to another given place.
Let A (Pl. 25. Fig. 8.) be the place from whence the corpuſcles diverge; B the place to
