If a ray pass through a body that refracts it from its original course, it will go on when it has got through, in a line parallel with its first direction.
Thus, let A B be a thick piece of glass, and C D, a ray of light passing through it, which would be refracted from c to d; C c would be exactly parallel to d D, and the point C would be seen from D as if it were at E.
If you think a little upon this, you will see that nothing is necessary to account for objects appearing nearer to a given point at one time than another, or objects upon the earth appearing high up in the air, except different degrees of refraction. Thus in the instance of the coast of France seeming to approach Hastings, it is evident that the effect would be produced by an extraordinary degree of refraction in a stratum of air over the sea, through which the rays of light producing vision must come.
The explanation will be the same as that of the diagram of the rising sun in a former page; if the ray from A took the direction D, as under ordinary circumstances it would do, it is evident that a spectator at C would see nothing. But instead of this, the ray was refracted near the middle to the point C, which represents the town of Hastings, and therefore the object A appeared as if it had been placed at B. You shall now hear what causes are likely to alter the refracting power of the air at times.
When air, water, or any other substance, is made hot, it becomes rarified, and its refracting power is thereby diminished. Have you ever watched, while you have been on one side of a stove, or of any heated body, the appearance of things on the other side through the air above it? If so, you must have seen how strangely they seem to tremble. Now this is caused by a stream of irregularly heated air rising from the stove, and enabling you every instant to see the things beyond it more nearly in their true position than you can through air of the ordinary temperature.
If you look through a magnifying glass at distant objects, they will appear upside down. You may learn why this takes place from books on optics. The same effect is produced by rays passing through a medium which becomes gradually denser, instead of suddenly passing from one state to another. Thus, if you take a square glass bottle and put some clear syrup into it, and then carefully pour water on the top of that, anything, such as a written or printed line, seen through the space where the liquids are mixing, will appear inverted.
Again, if you take a tin tube full of water, stopped with a piece of plain glass at each end, warm the middle of the tube, and then look at one end, you will see an object at the other end, if held at a proper distance, magnified, and distant objects turned upside down, just as they would be by a convex lens. If, on the contrary, you cool the middle of the tube, by applying ice to it, the same ensues as by using a concave lens. If a space of cold air be between two spaces of hot air, or the contrary, a space of hot air be between two spaces of cold air, the effects would be the same, only they could not be produced in so small a compass as they could with water. The space where the two different temperatures were gradually mixing, would influence the rays of light in the same manner as a lens on a very large scale.