side of the mass, so that its defects may be located and the perfect portions utilized to the best advantage. The mass is cut into slabs suitable for working up into prisms, lenses, and other optical instruments.
When a large disk is to be made, such as the great lens of a refracting astronomical telescope, several attempts are frequently necessary before success is gained. Two or even three years may pass before suitable material is cast.
As the result of this very troublesome process, we have slabs of fairly homogeneous glass from 3·4 to 3·6 times as heavy as water. Although the greater density is about equal to that of the diamond, the refractive indices of the two substances are not the same. That of the diamond is 2·5 and of the flint glass 1·61. But even this refractive power is the key to many mysteries. The trouble of producing the material counts as nothing in face of the results.
The possibility of increasing the refractive index of optical glass by increasing the density early attracted the attention of experimenters. Since the beginning of the century attempts have been made in this direction. By the use of the heavier rare metals, such as thallium, a glass has been produced over five times as heavy as water. The material has served admirably for the manufacture of artificial jewels, but has not as yet found permanent application in science.
The refractive power of glass, by which the rays of light are bent out of their course and images of objects formed, has its disadvantages as well as its merits. It is almost impossible to construct a lens which shall converge the rays of light without, at the same time, producing rainbow colors around the image. This defect is called chromatic aberration, and, as one can readily see, is fatal to the definition of the lens. It is commonly overcome by employing a compound lens, made of flint and crown glass. The different refractive indices of the materials correct each other's aberration and produce white light. A lens so constructed is termed achromatic, since it does away with the fringe of color. Loss of power is naturally the price of such a correction. These difficulties led to the project of making lenses out of a material which should obviate the color fringe by something in the glass itself. It is found that titanic and boric acids have a marked effect upon the refraction of the differently colored rays, and compounds of these materials have been used to good purpose. We have here a field well worth further exploration.
The best flint glass for optical use is made in Europe. It is an interesting circumstance that the great establishments of Messrs. Chance and Company, at Birmingham, and M. Feil et Cie.—now M. Mantois—at Paris, which largely supply the Ameri-
