Mrs. B. When parallel rays ([fig. 6]) fall on a double convex lens, that only, which falls in the direction of the axis of the lens, is perpendicular to the surface; the other rays, falling obliquely, are refracted towards the axis, and will meet at a point beyond the lens, called its focus.
Of the three rays, A B C, which fall on the lens D E, the rays A and C are refracted in their passage through it, to a, and c; and on quitting the lens, they undergo a second refraction in the same direction, which unites them with the ray B, at the focus F.
Emily. And what is the distance of the focus, from the surface of the lens?
Mrs. B. The focal distance depends both upon the form of the lens, and on the refracting power of the substance of which it is made: in a glass lens, both sides of which are equally convex, the focus is situated nearly at the centre of the sphere, of which the surface of the lens forms a portion; it is at the distance, therefore, of the radius of the sphere.
The property of those lenses which have a convex surface, is to collect the rays of light to a focus; and of those which have a concave surface, on the contrary, to disperse them. For the rays A and C, falling on the concave lens X Y, ([fig. 7, plate 19.]) instead of converging towards the ray B, in the axis of the lens, will each be attracted towards the thick edges of the lens, both on entering and quitting it, and will, therefore, by the first refraction, be made to diverge to a, c, and by the seconds, to d, e.
Caroline. And lenses which have one side flat, and the other convex, or concave, as A and B, ([fig. 1, plate 20.]) are, I suppose, less powerful in their refractions?
Mrs. B. Yes; the focus of the plano-convex, is at the distance of the diameter of a sphere, of which the convex surface of the lens, forms a portion; as represented in [figure 2, plate 20.] The three parallel rays, A B C, are brought to a focus by the plano-convex lens, X Y, at F.
Emily. You have not explained to us, Mrs. B., how the lens serves to magnify objects.