obtain the needed colors, and additions to the list are constantly being made as the result of further experimentation. Taking the colors up in the order of the spectrum, the violet shades are generally produced from manganese or from very small quantities of cobalt; the deep blues, indigos, purple blues, and normal blues, from varying proportions of cobalt; peacock-blue from copper; the finest greens from copper and chromium; and the dull seawater tint from ferrous oxide. The oxide of copper gives an emerald green. The yellows come from a variety of sources: the sesquioxide of uranium gives a fine fluorescent yellow; the oxide of lead a pale yellow, and the oxide of silver, applied as a pigment to the surface of the glass, a permanent yellow stain. The higher oxide of iron gives an orange color, but, as it has a strong tendency to become reduced, it is necessary during the manipulation of the glass to keep some oxidizing agent present, such as manganic oxide. In the reds a number of excellent shades are readily obtainable. Manganese furnishes a variety of pinkish reds and pinks; copper, in its lower oxide, the fine blood-red of Bohemian glass; and gold, the deepest and most brilliant of all reds, the well known ruby glass. This list, however, is but a fragment. It bears to the complete array of color at the command of the glass-worker much the same relation that an inventory of crude pigments would bear to the fine distinctions housed in an artistes color-box. It is only intended to give some little idea of the mineral bases utilized for their color effect. The fine gradations of color, and the rich and delicate tones, are the result of no such elementary chromatics. Many substances have joined their forces to produce these fine results. In many cases they have been obtained only after long experimentation, and have a corresponding value in the eyes of their discoverers. The magnificent window designed by Mr. John La Farge, which now faces the chancel in Trinity Church, Boston, owes the brilliancy of its peg, cock hues to the combined forces of some seventeen ingredients. This is an extreme instance of complexity, but it fairly represents the present tendency to secure a multitude of effects even at the expenditure of a multitude of agents.
In addition to these metallic compounds a number of other substances are used to produce either colors or unique effect. A little carbonaceous matter yields an amber tint of very agreeable hue, while the opalescence now so much in vogue and so justly admired results from the presence of oxide of tin, arsenic, or lime, or from native minerals, such as fluorite or the cryolite now imported in such large quantities from Greenland. The superiority of American art-work in glass is largely due to the introduction of this effective opalescent glass. It was first used in this country about ten years ago, by Mr. John La Farge and Mr. Louis C.