Some slight knowledge of the refraction of light may be of great importance to boys or lady-bathers. We shall subjoin a few remarks from "Cyclopaedic Science," and a practical hint on the subject: -

"When a ray of light passes from one medium to another of the same density, and in a perfectly straight line, no alteration of its course takes place; but if the light passes in an oblique direction, its course is broken or refracted - i.e., bent back from its natural path. To this branch of optics, which includes, perhaps, the widest field of inquiry, and traces the propagation of light through transparent, solid, liquid, and gaseous bodies, has been given the name of Dioptrics.

" To prove that a straight line representing a ray of light is really bent when passing from a rare medium, air, into a denser one, such as water, nothing is easier than to place a bright shilling on the end of an ivory paper-knife, which is inclined in a large empty tumbler. On looking down the paper-knife a straight line only is apparent, terminating with the coin; but if the tumbler is filled with water whilst the observer is still looking down the flat surface, he will notice that at the point of juncture between the air and water a break takes place, and the end of the paper-knife, or all that part immersed, appears to be lifted up or bent upwards from its natural course or direction. If a small pocket-pistol were now aimed at the coin and the bullet discharged it would certainly miss, because every visible object appears to be in a direction represented by a straight line drawn from it to the eye. A straight line ruled to the shilling would not touch it, the line must be ruled to, or the pistol aimed at, a point nearer to the spectator than the apparent position of the coin".

This refraction of light causes a river or stream to appear much more shallow than it really is, because the light of the bottom of the river is refracted as it comes out of the water, and the bottom consequently appears much nearer to the surface than it really is. The old "Sandford and Merton" experiment of placing a shilling in a bowl and moving away till you lose sight of it, and then getting some one to pour water in on it, which brings it to your view again immediately, by refraction, will exhibit this delusion, which it is well for boys and young-lady swimmers to know. A river is always one-third deeper than it seems to be.