Not only to heat but to light Jena glass renders a service. Glass of ordinary kinds when used for the tubes of a Hewitt mercury-vapor lamp, absorbs a considerable part of the ultra-violet rays upon which photography chiefly depends. A Jena glass free from this fault is formed into Uviol lamps of great value in taking photographs, photo-copying, and photo-engraving. These lamps are also employed in ascertaining the comparative stability of inks and artificial dyes; so intense is their action that brief periods suffice for the tests. Uviol rays severely irritate the eyes and skin; they may prove useful in treating skin diseases. They moreover quickly destroy germs. In all these activities reminding us of radium. Thus by a bold departure from traditional methods in glass-making, the eye receives aid from lenses more powerful and more nearly true than ever before swept the canopy of heaven, or peered into the structure of minutest life. Meanwhile instruments of measurement take on a new accuracy and retain it as long as they last. All this while a material invaluable for its transparency is redeemed from brittleness and corrodibility, and given a strength all but metallic; at the same time transmitting light with none of the usual subtraction from its beams. [Illustration: Bliss forming die. A, bed plate. B, blank-holder. C, drawing punch. D, push-out plate. O, P, annular pressure surfaces.] [Illustration: Bliss process of shell making.] Power Presses in Metal Working. From glass let us now turn to metals. It is their tenacity that chiefly gives them value; this tenacity is usually accompanied by a hardness which disposes us to regard nickel, for example, as of a solidity quite unyielding. But the coins in our pockets prove that under the pressure of minting machinery they are as impressible as wax. In molds and dies, each the counterpart of the other, brass, bronze, iron, steel, and tin-plate take desired forms as readily as if paste. Solid though these metals appear they yield under severe stress with a semi-fluid quality. We have long had stamped kitchen ware, baking pans, and the like; the principle of their manufacture has of late years been extended to ware of more importance. Bliss power presses are to-day turning out hundreds of articles which until recently were either slowly hammered or spun into form, pieced with solder, or shaped by the gear cutter or the milling machine. These presses furnish the United States Navy with sharp-pointed projectiles, some of them so large as to demand a million pounds pressure for their production; they make strong seamless drawn bottles, cylindrical tanks for compressed air and other gases, and cream separators able to withstand the bursting tendency of extremely swift rotation. [Illustration: Mandolin pressed in aluminium.] [Illustration: Pressed Seamless pitcher.] [Illustration: Barrel of pressed steel.]