To continue the subject of lenses, in order that our readers may fully understand it, we present an engraving of a double-convex lens and it will be seen that the focus is nearly in the center—in fact, exactly where we described it last week. The following figure will show the general principle on which a a convex lens magnifies : — Let A E c D be an object, seen by an eye at E, through the lens, t, 8. The angle, BED, is the natural angle under which the object would be seen if the lens were not there. But when tbe rays, n E and D u, arrive at the points, x x, they are refracted nearer to the perpendicular, and would meet in some point, 3, nearer the object than the eye is ; and if the eye were not moved to that point, the object would not be distinctly seen. But, if the side of the lens toward the eye is also convex, as in the figure, the rays, as 1, x 1, will, in passing out of the denser convex medium into the rarer concave one at the point,, r r, be again refracted towards the axis, E c, and will meet at some point, 1, between the lens and 3 ; so that, in order that the olijcct may be seen, the eye must be moved to the point, 1. The ray which is made up of the three parts, r, x, x v, and v ], will appear to come in the last direction, that is, in the direction v 1, because the direction in which it falls on the eye is the only means that we can have of knowing the direction of light. But the direction of every ray from the object will be altered in the same proportion, and the outline through the len will be the magnified outline, abc ; and if it be an object of which a particular magnitude has not been fixed by experience, it will seem magnified to that extent. But if it were one with the dimensions of which the observer was so familiar, that he assigned it the same magnitude whatever the distance were, then the impression would be that it had been brought nearer to the eye, as in the dotted lines, 5 c d. The following figure will in like manner illustrate the effect of concave lenses :— Let A Bbean object, seen by an eye at E through the concave leuse, i, s. A F. B would be the natural visual angle, if the lens were not there. But the rays, A X B X, falling upon the concave surface, x x, are refracted from the perpendicular, in the directions, xvx v, and would meet at the same point, y, behind the eye. But as they again fall upon the con-i vex surface of the rarer medium (or, which is he same thing, pass out of the conaave surface of the denser one) at v and v, they are again refracted from the perpendicular, and w ill meet at the same point, L, behind y. They will have the last directions, v z, v z, so that in order to see them, the eye must be removed back to L, and the object will be diminished to the size, a b, or if size has been associated with it, it appears as if removed to the position of the distant a b