reagents, make it one of the most useful of servants in the good cause of science.
First, then, a word in regard to its refractive power.
If a beam of light pass from one medium to another of different density, such as from air to water, its course is not altered, provided the surfaces of the two media be at right angles to the beam. A penny placed in a basin of water looks in no way distorted if the eye be directly above the coin. But when a beam of light passes into a second medium at other than a right angle its course is bent. A straight stick, partly immersed in water, looks crooked because the light reflected from the portion beneath the water is bent on entering the air. The fact is familiar to every one. This bending of the light has received the name of refraction, and its laws are exceedingly simple. If the beam pass into a denser medium, as from air into water, the bending is toward the perpendicular to the common surface of the two media. On the other hand, if the passage be into a rarer medium, as from water into air, the beam is bent away from the common perpendicular. We may, then, predict in a general way the course of a beam of light when it changes its medium, but in scientific work we must do better than that—we must know the exact course of the beam. This brings us to the second law of refraction, which is quite as simple as the first, but which requires, if one is not mathematically inclined, a trifle more patience for its comprehension. In any angle, if a perpendicular be dropped from any point on one side to the opposite side, the ratio between the perpendicular and the distance of its starting-point from the apex of the angle will evidently be a constant quantity for that angle, wherever the point be taken. This ratio is called the sine of the angle. If one will take the trouble to draw a series of angles from zero to ninety degrees, he will readily see that the value of the sine increases from zero to unity, and that these are its limits.
Now, it is found by experiment that the ratio between the sine of the angle of incidence (the angle which the impinging ray makes with the common perpendicular to the two media) and the sine of the angle of refraction (the angle which the refracted ray makes with the common perpendicular) is a constant quantity. This quantity is known as the index of refraction.
But it may be asked what all this has to do with glass-making. Essayists are prone to talk about evolution and the fourth dimension of space, and many other things which seemingly have no connection with the subject in hand. In this case, however, the wandering is justifiable, for the index of refraction is a constant which must ever be borne in mind by the scientific glass-worker, if he wishes to use the material in the construction of optical in-
