378

Sir Isaac Newton’s Book III.

'i. It will be under flood from what has been faid above,that when light falls upon the furface of glafs obliquely, afterits entrance into the glafs it is more inclined to the linedrawn through the point of incidence perpendicular to thatfurface, than before. Suppofe a ray of light bluing from thepoint A (in fig. 136) fills on a piece of glafs BCDE, wholelurface BC, whereon the ray falls, is of a fpherical or globu-lar figure, the center whereof is F. Let the ray proceed inthe line A G falling on the furface B C in the point G, anddrawF G H. Here the ray after its entrance into the glafs willpals on in fome line, as G I, more inclined toward the line F G Hthat the line A G is inclined thereto; for the line F G H is per-pendicular to the furface B C in the point G. By this means,if a number of rays proceeding from any one pointfall on a convex fpherical furface of glafs, they fhall beinfleCted (as is reprefented in fig. 1 3 7,) fo as to be gatheredpretty clofe together about the line drawn through the centerof the glafs from the point, whence the rays proceed; whichline henceforward welhall call the axis of the glafs: or thepoint from whence the rays proceed may be fo near the glafi>that the rays fhall after entring the glafs flill go on to fpreadthemfelves, but not fo much as before lo that if the rayswere to be continued backward (as in fig. 13 8,) they fhouldgather together about the axis at a place more remote fromthe glafs, than the point is, whence they actually proceed. Inthefe and the following figures A denotes the point to whichthe rays are related before refraction, Bthe point to which theyare directed afterwards, and C the center of the refradting fur-face. Here we may obferve, that it is pofiible to form the glafs offuch a figure, that all the rays which proceed from one point

fhall