struments. In general, the greater the index of refraction, the more available the glass. The practical question with him is to know the conditions which affect the index of refraction. To answer this intelligently, one must consider why the light is bent at all in changing its medium. If a bather run down a smooth, hard beach into the water, he is very apt to fall head foremost when he reaches the denser medium. His feet are suddenly retarded, while his body keeps on through the air with the old velocity. The result is a change of direction in his motion, which is in one sense disastrous.
Precisely the same thing happens to the light. It is generally conceded to be a progressive wave-motion. When the beam passes into a denser medium at an angle, the side of the beam which enters first is retarded, while the other side keeps on at the old velocity. The result is, that the whole beam is swung out of line and takes a new direction in the new medium. The index of refraction is simply a quantitative expression for this bending, and depends upon the nature of the substance and its density. The great brilliancy of the diamond is due to its very high refractive index, and the sparkle of cut glass is the result of a similar property.
Since the employment of glass in optics depends upon its ability to bend the rays of light to a common point or focus, its value increases with its refractive power—that is to say, with its density. The problem set before the maker of optical glass is, therefore, quite different from that which must be solved by the manufacturer of more every-day goods. He must produce a glass which has great weight without any loss of transparency. The difficulty lies in this, that the substances which add weight to the glass are prejudicial to its transparency. Success is found in the nice balance between these opposing tendencies.
Glass is a double silicate. If it is to have large density, the metallic bases combined with the silica must be heavy. Hence, the ordinary glass of commerce—a double silicate of lime and soda—will not serve in optics. In place of this, a double silicate of lead and potash must be used. The lead gives density to the glass, and consequently high refractive power. The crude materials must be as pure as practicable. To about a hundred parts of sand there is added a mixture of one hundred parts of minium, or red lead, and thirty parts of potash. When these are fused together in large, hooded crucible pots, a very liquid glass results. It is considerably more fusible than the lime-soda glass. So far, the process is easy; but the silicate of lead is so much heavier than the silicate of potash that when the fused mass is allowed to cool the denser silicate has a decided tendency to separate out at the bottom of the crucible. This makes the glass streaky and
