Page:The wonders of optics (1869).djvu/137

Rh the focus, in accordance with the law of refraction, by which a ray of light passing from one transparent medium, such as air, to another which in this instance is glass, becomes refracted or bent in proportion to the

relative density of the two mediæ. The nearer the ray passes to the edge of the lens, the more it is refracted, the angle of incidence being greater; the ray through the exact centre being uninfluenced by the form of the glass. Hence they all meet in a single point. Figs. 29 and 30 show the path of the rays when they are divergent and convergent.

If the rays of light are not parallel, as in the case of the source of light being near the lens, they do not converge so rapidly as when they proceed from a distant object, consequently the focus for near objects is longer in proportion to their distance. In fig. 29 for