Jena Glass.
As in the case of the aluminium bronzes and nickel steels, alloys of the utmost value have been formed by introducing new ingredients, often in little more than traces, or by modifying but slightly the proportions in which ingredients long familiar have been mingled together. An equal gain has followed upon varying anew the composition of glass. For centuries the only materials added to sand for its melting pot were silicic acid, potash, soda, lead-oxide, and lime. As optical research grew more exacting the question arose, Will new ingredients give us lenses of better qualities? First of all came the demand for glasses which combined in lenses would yield images in the telescope and microscope free from color. In a simple lens, such as that of an ordinary reading glass, we can readily observe the production of color by a common beam of light. The rays of different colors, which make up white light, are refrangible in different degrees, so that while the violet rays come to a focus near the lens, the red rays have their focus farther off; the images, therefore, instead of being sharply defined, are surrounded by faint colored rings. In a telescope or microscope a simple lens would be of no value from the indistinctness of its images. To correct this dispersion of color a second lens of opposite action is placed behind the first, that is, a crown-glass lens is added to a flint-glass lens. (See [cut], p. 255.) This remedy is not quite perfect for the reason that the distribution of the spectrum from violet to red varies with each kind of glass, and in such a way that through failure of correspondence, color to color, in a compound lens, variegated fringes of light, though faint, are perceptible, much to the annoyance of the microscopist, the astronomer, and the photographer.
With a view to producing glasses which united in compound lenses should be color free, Rev. Vernon Harcourt, an English clergyman, in 1834 began experiments which he continued for twenty-five years. By using boron and titanium in addition to ordinary ingredients of glass, he produced lenses less troubled by color than any that had before been made. His labors, only in part successful, were in 1881 followed by those of Professor Ernst Abbe and Dr. Otto Schott at Jena. With resources provided by the Government of Prussia, these investigators were able to do more for the science and art of glass-making than all the workers who stood between them and the first melters of sand and soda. They immensely diversified the ingredients employed, carefully noting the behavior of each new glass, how much light it absorbed, how it behaved in damp air, what strength it had, how it retained its original qualities during months of keeping, and in particular how variously colored rays were distributed throughout its field of dispersion. As in the blending of new alloys it was found that many of these novel combinations were useless. Of the scores of new glasses produced some were extremely brittle, others were easily tarnished by air, or so soft as to refuse to be shaped as prisms or ground as lenses. A more systematic plan of experiment was therefore adopted: for the production of new glasses there were by degrees separately introduced in varied quantities, carefully measured, boron, phosphorus, lithium, magnesium, zinc, cadmium, barium, strontium, aluminium, berylium, iron, manganese, cerium, didymium, erbium, silver, mercury, thallium, bismuth, antimony, arsenic, molybdenum, niobium, tungsten, tin, titanium, fluorine, uranium. An early and cardinal discovery was that the relation between refraction and dispersion may be varied almost at will. For example, boron lengthens the red end of the spectrum relatively to the blue; while fluorine, potassium, and sodium have the opposite effect. With the distribution of the diverse hues of the spectrum thus brought under control, there were produced glasses which, when united as compound lenses, were almost perfectly color-free, rendering images with a new sharpness of definition. Yet more: in their unceasing round of experiments Professor Abbe and Dr. Schott came upon glass so little absorbent of light that combinations of much thickness intercepted only a small fraction of a beam; they were indeed almost perfectly transparent. This achievement is of great importance to the photographer, whose planar combination of six lenses may be four inches in thickness. At Jena the researchers are endeavoring to perfect another gift for the camera: they seek to produce glasses each transmitting but one color, for service in color-photography.
To microscopy they have recently given lenses which completely transmit ultra-violet rays so as to photograph the diffraction discs of objects, such as gold particles in colloidal solutions, otherwise invisible, because below the resolving power of the most powerful microscope. It is estimated that with this new aid an object but 1⁄250,000,000 of a millimeter in length may indirectly be brought to view.
One ancient art, that of annealing glass, Professor Abbe and Dr. Schott greatly improved, eliminating from their products the stresses which distort an image. By means of an automatic heat-regulator, the temperature of a batch of glass could be kept steadily for any desired period at any point between 350° and 477° C.; or allowed to fall uniformly at any prescribed rate. The glass was usually contained in a very thick cylindrical copper vessel, on which played a large gas flame. The highest temperature found necessary to banish stress, that is, to cause softening to begin, was 465° C. The lowest temperature required to ensure complete hardening was about 370° C. Thus the temperatures of solidification all lie between 370° and 465°. This fall of 95° was spread over an interval of four weeks, instead of a few days as formerly, with the result that stress was banished utterly.
Photograph by Bräunlich & Tesch.
THE LATE PROFESSOR ERNEST ABBE, OF JENA.
A practical example of the benefits gained in the properties of Jena glass is exhibited by its use in measuring heat. A thermometer of common glass when first manufactured may tell the truth, and in a month or two may vary from truth so much as to be worthless. The reason is that the dimensions of the glass slowly change day by day, as in a less degree do those of many alloys. It was one of the aims of the Jena laboratory to produce a glass which should remain constant in its dimensions while exposed to varying temperatures, so that, made into thermometers, it would be thoroughly trustworthy. Here, too, success was attained, so that thermometers of Jena glass are found to be reliable as are no instruments of ordinary glass. This product is available for astronomical lenses, otherwise liable to serious changes of form as exposed successively to warmth and cold.
Heat was to be staunchly withstood not only in moderate variations, but in extreme degrees. From time immemorial heat suddenly applied to glass has riven it in pieces. Could art dismiss this ancient fault? To-day a beaker from Jena may be filled with ice and placed with safety on a gas flame. In its many varieties this glass furnishes the chemist with clean, transparent and untarnishing vessels for the delicate tasks of the laboratory, all of singular indifference to heat and cold. Yet again. Special kinds of this glass in chemical uses are attacked by cold or hot corrosive liquids only one-twelfth to one-fourth as much as good Bohemian glass, the next best material.
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.
Bliss forming die. A, bed plate. B, blank-holder. C, drawing punch. D, push-out plate. O, P, annular pressure surfaces.
Bliss process of shell making.