Magnifying glass

“ Often it takes a whole year, and sometimes even longer, to polish a large telescope lens. Without this magnifying agency we should have no astronomy, and fewer scientific discoveries than we now have. The glasses people wear all have to be ground and polished in much the same fashion; opera glasses, magic lanterns, and every contrivance for bringing distant objects nearer or making them larger are dependent for their power upon glass lenses."
"Even when making glass I never dreamed it could be used for so many different purposes," answered Giusippe. ”

“ If a glass shows a colored fringe around a bright object, reject it. Let the diameter of the object-glasses, which are the large lenses in the end farthest from the eye, be not less than an inch and a half. The magnifying power should be at least three or four diameters. A familiar way of estimating the magnifying power is by looking at a brick wall through one barrel of the opera-glass with one eye, while the other eye sees the wall without the intervention of the glass. Then notice how many bricks seen by the naked eye are required to equal in thickness one brick seen through the glass. ”

“ With equal object-glasses they have a greater magnifying power, because the luminous rays that traverse the glasses lose less by absorption than the reflection on the metallic mirror of telescopes; but the thickness that can be given to glass is limited, for too thick it does not allow the luminous rays to pass. ”

“ Of course the more intricate the design the more numerous the processes. The expert uses magnifying-glasses, but is said to depend more on the delicacy of his own sense of touch than on the power of the glasses. It is scarcely possible to imagine a higher effort of hand and eye than this nunome-zogan displays, for while intricacy and elaborateness are carried to the very extreme, absolutely mechanical accuracy is obtained. Sometimes into the same design gold enters in three different hues, obtained by varying the alloy. ”

“ In the seventeenth century the whole process was revolutionised by the introduction of a large proportion of oxide of lead, making what is technically known as “flint” glass—a glass much more brilliant than any other, a quality due partly to its transparency and partly to its increased refractive power, which renders it specially fitted for “cutting”—a process which enhances its beauty by increasing the number of ways in which the light rays falling on the glass are dispersed. ”

“ For no other material becomes so hot, or bears the fire for such a convenient time, until the material of the glass is perfectly fluid, and is at the same time burnt up by that ardent fire. It happens, however, sometimes, that on account of the magnetick stone, the magnesia, or the ore, or the pyrites, the glass has a dusky colour, when they resist the fire too much and are not burnt up, or are put in in too great quantity. ”

“ We here see that lead-glass or flint-glass has little greater refractive power on light than rock crystal or the ordinary plate and crown glass of commerce which belongs to the same family as the cristallo of the Venetians. In dispersive power, on the other hand, it stands apart from both these substances and rivals the diamond in scattering the component rays of white light. ”

“ It is by their aid we get those mosaics of pot-metal, the depth and richness of which to this day, with all our science of chemistry, we cannot approach by any process of enamelling. Moreover, though merely constructional leads, taking a direction contrary to the design, may at times disturb the eye, (they scarcely ever disturb the effect) they add to the richness of the glass in a way its unlearned admirers little dream. ”

“ Considerable curiosity has been excited of late by a new invention, which has resulted from the investigations of a Frenchman. We have been told that tumblers and wine-glasses, and other glass utensils, could be so treated that they would never break; and experiments performed upon many samples of these glasses led one to suppose, that the object had been attained. There is no doubt whatever, that some who have had experience of what is termed toughened glass know, that in many cases very uncertain results are obtained in the resisting power of the glass to the action of a violent blow. ”

“ When therefore the objects are light on dark, the lead-work will sink into the background, and although it may leave small space for the glass, it is sometimes surprising how efficiently that little lights up and expresses the colour. If the charges are inconveniently small for the leading, resort is had to what is called flash glass, which consists of two layers, of which one only is coloured, and is made by dipping a piece of molten white into a coloured glass, when the mass is about to be blown into the bulb which, shaped and expanded, ultimately becomes a sheet of glass. ”

“ So, too, we have only to turn the glass before laying it on the paper, and we bring all the natural relations of the object delineated back again,—its right to the right of the picture, its left to the picture's left.
On examining the glass negative by transmitted light with a power of a hundred diameters, we observe minute granules, whether crystalline or not we cannot say, very similar to those described in the account of the daguerreotype. But now their effect is reversed. Being opaque, they darken the glass wherever they are accumulated, just as the snow darkens our skylights. ”

“ Some imagined that they had found magnets for gold and silver. In the oldest times men had so erroneous an opinion of the art of glass-making, that they conceived that glass was obtained from sand, as metal from its ore; and Pliny thinks that they then conjectured that a magnet could attract glass as well as it does iron. Now as manganese, on account of its similarity, was considered to be a magnet, it was consequently subjected to experiments, which gave rise to the beneficial discovery that it renders glass colourless. ”

“ There remains, in the form either of iridescent scales or of an opaque pearly crust, a layer consisting not perhaps of pure silica, but of an acid silicate of lime, alumina, or lead as the case may be.
Now a piece of clear glass may appear to the eye to be devoid of internal structure. But the ‘metal’ has, we know, in every case been subjected during the manufacture to a complicated series of involutions and doublings, to say nothing of the subsequent inflation if the glass has been subjected to a blowing process. ”

“ The action of peroxide of manganese (the only compound of the metal used in the manufacture of glass) is simple and clearly understood. The sand (silica) used in the manufacture of glass frequently contains iron, which by the heat necessary for the fusion of the glass becomes reduced to the state of protoxide, giving the glass a greenish or yellowish colour; also, if any organic substance be present in the materials (and where sulphate of soda is used, charcoal is added) , the glass is not colourless. ”

“ On the other hand, glasses very rich in lead are always decidedly yellow in colour, and it has been shown that this coloration is due to the natural colour of lead silicates and not to the presence of impurities. What has just been said of lead applies, with only very slight modification, also to the rare metal thallium and its compounds, which have been introduced into glass for special purposes. Leaving these two exceptional bodies on one side, we now pass to a consideration of the elements in the order of their chemical grouping. ”

“ The instant the mass of soft metal was on the table a roller of cast-iron was passed very swiftly back and forth over it, spreading it to uniform thickness, and at the same time flattening it.
"The thickness of the glass is gauged by the strips of iron on which the roller moves," explained Giusippe to Jean. "These can be adjusted to any thickness. Notice how rapidly the men have to work. The glass must be finished while it is hot, or there will be flaws in it. ”

“ If, instead of oxide of lead, lime be mixed with carbonate of soda and sand, and the mixture be heated to a high temperature, a glass will be formed, in many respects similar to that of which oxide of lead is a constituent, but differing from it in several important particulars. First of all, the lead glass is highly lustrous, and has a great power of refracting light, so that, when it is cut, it presents a brilliant appearance, and by refraction readily produces the prismatic colours. ”

“ Glass is made into vessels, as most people know, by "blowing." The workman takes a dab of molten glass on the end of a long metal pipe, and putting his mouth to the pipe blows the glass—soap-bubble fashion—into a hollow bulb. Then by a rapid and dexterous series of alternate and repeated heating, blowing, and spinning, and 15 manipulation with tools, most fascinating to watch, he shapes it into the form required. If a flat sheet be required for window glass it may be run out flat when liquid, or blown as described above, and worked into a cylindrical form, split open, and unrolled. ”

“ The latter kind is known as “flashed,” and is universally employed in the case of colouring matters whose effect is so intense that in any usual thickness of glass they would cause almost entire opacity. Flashed glass is produced by taking either the first or the last gathering in the production of a cylinder out of a crucible containing the coloured “metal,” the other gatherings being taken out of ordinary white sheet-glass. ”

“ The only other base having any considerable commercial importance in connection with glass-making is barium oxide, while oxide of zinc, magnesia, and a few other substances are used in the manufacture of special glasses for scientific, optical or technical purposes, where glass of special properties is required. ”

“ The softness of lead which makes it the only practicable metal for joining pieces of glass of complicated shapes, has the disadvantage that a stained-glass window when leaded up has a considerable degree of flexibility, and, if held by the edges alone, would be quite unable to resist the pressure of the wind, which on a big window is enormous,—think of the power even of a fresh breeze on a boat's sail. ”

“ The final stage with which we are concerned is that of blowing, since all table glass, worthy of being called table glass, is blown. In other words, every decanter, vase, tumbler, and wine glass of the better sort begins its existence as a bubble of molten glass at the end of an iron tube—the glass-blower’s tube—and owes its form to the delicate touches of simple tools held in a skilful hand and guided by a trained eye. It is this fact which gives glass its individuality. There is no hard-and-fast rigour of line, no mechanical uniformity of shape, such as is associated with machine-made goods ”

“ There is a late example of this form at South Kensington and another in the British Museum. The stechel-nuppen may assume less aggressive forms; the points may be smoothed down while the metal is soft, and we then have merely a series of disc-like thickenings on the sides of the glass. By this means, as in the more refined Dutch roemer of 254 the seventeenth century, effects of great beauty, due to the varying transparency of the glass, were obtained. ”

“ Points to be considered are the colour of the metal, the perfection of the twist, its opacity, and the character of the foot. The colour and weight of the glass are probably the best criteria. The forgeries are generally light, and the beautiful mellow tone of the real old English glass is replaced by a cold white tint or even a tinge of green. The twist is often more translucent than in the genuine pieces, and is frequently imperfectly produced, becoming looser as it descends, the loosening being particularly pronounced just before its junction with the foot. ”

“ In what is now known as the older crown and flint glass the dispersion and refraction increased with the density, that is to say, the heavier the glass the more it scatters and bends light rays passing through it. With the new methods, glass is made which scatters the light rays very little, though bending them considerably and vice versa. The ordinary crown glass is composed of silicates of calcium and sodium or of calcium and potassium or a mixture of both and it is possible to make it colourless and free from defects, but its optical properties are never so valuable as those of Jena glass. ”

“ A window of ancient glass needs releading. The lead has decayed and the whole is loose and shaky. The ancient glass has worn very thin, pitted almost through like a worn-out thimble with little holes where the alkalis have worked their way out. It is as fragile and tender as an old oil-painting that needs to be taken off a rotten canvas and re-lined. If you examine a piece of old glass whose lead has had time to decay, you will find [Pg 316] that the glass itself is often in an equally tender state. ”

“ This “covering” of the greenish tinge is generally most completely successful in the case of soda-flint glasses, but the method is also used to a certain extent in the case of the soda-lime glasses used for sheet and plate-glass manufacture. Manganese added to glass for this purpose is generally introduced into the mixture in the form of the powdered black oxide (manganese dioxide) , which is available as a natural ore in a condition195 of sufficient purity. ”

“ It always happens that for some special wave-lengths the substance has the power of absorbing the energy of the entering waves and converting it into heat-vibrations of its own molecules or atoms. In the most transparent and colourless glasses this process, so far as the waves of ordinary light are concerned, only goes on to a negligibly slight extent; if, however, we extend our view beyond the range of ordinary visible light, and consider the region of shorter waves that lies in the spectrum beyond the violet, we find that ordinary colourless glass becomes strongly absorbent ”