Having described all the ordinary uses of the microscope and having also insisted that the objectives are the most important part of the instrument, there are probably many of our readers who may wish to know in what manner this wonderful glass differs from ordinary glass and how it is made.
Glass has been defined as a substance which, during its manufacture, passes from the liquid to the solid state so rapidly that no crystals are formed. Usually when solids are melted and then allowed to cool, they do so with the formation of crystals, this may be shown in the case of sulphur by melting a little and then allowing it to cool. After a while, as cooling takes place a solid crust will be formed on the surface of the molten sulphur. If two holes are pierced in the crust and the still liquid sulphur poured out, it will be found that the sulphur which adhered to the vessel in which the melting took place has formed beautiful needle shaped crystals.
A great amount of original work has been done on the subject of glass manufacture and especially on the kind used for optical instruments—as a result there are many kinds of glass differing from one another in physical properties and in chemical composition. Although the various chemicals used and their proportions are fairly well standardised, as the result of long experience, it is probable that glass is not a definite chemical compound but a mixture, in which certain of the components act as solvents for the rest.
The ordinary glass in use in this country, apart from specially prepared optical glass, may be either English flint glass, plate glass or Bohemian glass. The first named is composed of sand, potassium carbonate and red lead; plate glass is made of sand with the carbonates of sodium and calcium, whilst similar ingredients are used for Bohemian glass except that carbonate of potassium is substituted for carbonate of sodium. It is chiefly owing to the requirements of optical instrument makers that, new kinds of glass containing very many previously untried chemicals, have been produced. As a result glasses are now made with specific gravities varying from 2.5 to 5.0, that is to say the weight of a square inch, or a square foot or a square yard of glass may weigh anything from two and a half to five times more than a square inch, foot or yard of water, according to its composition.
Before we describe the details of its manufacture let us consider its properties as briefly as possible. At high temperatures it is perfectly fluid and may be poured from vessel to vessel as easily as water; at lower temperatures it is viscous, i.e., semi-fluid and can be rolled with an iron roller as dough is rolled with a rolling pin; it can be moulded into any desired shape, blown out into flasks and bottles or drawn out into threads so fine that they may be woven into a fabric. It is a bad conductor of heat and for this reason it is safer to pour very hot liquids into a thin glass vessel than into a thick one. With thick glass the inner layers expand with the heat before the outer layers are even warm and the result is a crack or often absolute fracture. Sometimes during manufacture glass vessels which are suddenly cooled will appear satisfactory, but the particles of glass remain in so high a state of tension that at the slightest touch the vessel will break up into thousands of pieces. On account of this property of glass it must be cooled very slowly indeed; the process is known as annealing.
Optical glass unlike most other kinds must be manufactured in thick blocks—some of the large lenses on telescopes are of considerable thickness. All glass for scientific instruments must also be homogeneous, which our dictionary tells us means of the same kind. To be more explicit each particle of optical glass should be precisely the same in composition and properties as every other particle. In the very early days of manufacture it was difficult to obtain homogeneous pieces of glass and Guinard, in the 18th century, conceived the idea of stirring the molten glass with a rod of fireclay, to ensure a thorough mixing of the components. This led to considerable improvement and the method has survived to the present day.
The first real advance in the manufacture of optical glass, was due to the ingenuity of two Germans, Abbe and Schott, who lived at Jena. Jena glass became famous for the manufacture of lenses, so much so that a stupid idea still prevails in many quarters that only the Germans can make good optical glass. To give them their due it is good but quite recent events have shown the world that the Britisher can make better. The two German scientists used new chemicals in making their glass and they succeeded in producing a substance which possessed hitherto unheard of properties. 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. The most important components of the newer glass are the oxides of Barium, Magnesium, Aluminium, Zinc and Boron.
Good optical glass should be transparent and colourless and, as we have stated, it should be homogeneous—the refraction and dispersion of light rays should be identical over all parts of the glass. It should possess no striæ, as they are called. Striæ may be seen at the edge of a piece of plate glass as little lines just as though the glass had been formed in layers. Striæ detract from the efficiency of optical glass, nevertheless, some very cheap lenses are made of plate glass. Bubbles are almost always present in Jena glass but, unless they are very numerous they do not appear to render the glass less efficient. Hardness and chemical stability are other desirable qualities. Most of these high-grade glasses are soft, as shown by the ease with which they may be scratched; many of them are not very stable chemically and are easily affected by chemical fumes with the result that their surfaces become covered with a coloured film. With all their drawbacks, for optical work the newer glasses far excel the older.
The manufacture of optical glass is a costly and lengthy process. The chemicals used in its manufacture are selected with the greatest care; impurities must be guarded against for they would change the composition of the glass and in doing so alter its physical properties on which everything depends. The chemical substances are used either in the form of oxides, nitrates or carbonates, for the reason that they are easily decomposed by heat. To assist in the melting of the substances a few fragments of glass of similar composition are added. The crucibles, in which the chemical components are melted, are covered so that no fumes from the furnace may gain access to them, even the chemical composition of the crucibles is carefully tested that no impurities may contaminate the glass. No crucible is ever used more than once and only a single crucible is heated in each furnace, in order that the temperature may be regulated to a nicety.
The actual manufacture is then begun after all these preliminaries have been attended to. A clean dry crucible is heated in a furnace—not the one in which the glass making is to take place—to a dull red heat. Then, with iron tongs, it is removed to the previously heated glass making furnace and the temperature is raised very gradually. The next stage sees the addition of the well mixed chemicals to the heated crucible, in small quantities at a time. When the full quantity has been added the crucible contains melted glass full of bubbles, some of them air bubbles released from the raw materials as they were added and some bubbles of gas given off from the chemicals as they act upon one another. The molten glass is then heated strongly so that it will become perfectly liquid and many of the bubbles will be driven off. To reach this stage may occupy anything from thirty-six to sixty hours and constant attention is necessary during the whole time.
The next stage is perhaps the most important in the whole process. After numerous small samples of the molten glass have been taken, on the end of iron rods, to see if the air bubbles have been driven off, the mixture is stirred to render it homogeneous and to eradicate striæ. The stirring is carried out by means of a cylinder of fireclay which is first of all heated to the temperature of the molten glass before it is introduced. To the end of the fireclay cylinder a long, detachable iron handle is fixed so that the man who undertakes the stirring may stand at a distance from the hot furnace. The heat is great and the work of stirring is laborious and for this reason the stirrers are constantly changed. The iron handles must be watched carefully for, owing to the heat they rust rapidly and should any of the rust fall into the molten glass it would impart to it a colour and render it useless for optical purposes. When stirring begins the glass is liquid as water, but the stirring is continued during cooling and all the while the glass is gradually becoming more and more solid. During the final stages the operation is hard labour indeed and, finally, it is not possible to stir any more, then the fireclay cylinder is either removed or left in the glass.
When the glass has solidified and, whilst it is still hot the furnace is sealed and allowed to cool very gradually till it has reached the ordinary temperature of its surroundings; this may take several weeks. When quite cool, the crucible is removed from the furnace and carefully broken; then the glass, which may be in one mass weighing as much as 1000 lbs., is freed from particles of fireclay and examined for defects.
The next stage consists of moulding and annealing. Large pieces of glass are heated till they are just soft, then they are passed into iron or fireclay moulds designed so that the glass is formed into discs or slabs suitable for grinding by opticians. They are allowed to cool very slowly in the moulds.
When the glass is taken from the moulds it is not yet ready to be made into lenses. It is subjected to another and very careful examination, when all defective parts are cut out. Should there be many defects the glass is again heated, moulded and annealed. From a crucible containing 1000 lbs. of molten glass it is unusual to obtain much more than 200 lbs. of optically perfect glass, it is obvious therefore that lenses cannot be cheap.
One would naturally imagine that the minute lenses used in microscope objectives should be cheaper in comparison than the larger lenses used for photography or in telescopes, for it is always more difficult to make minute articles than large ones. As a matter of fact, even allowing for the greater amount of material used in the larger lenses, quality being the same in both cases, they are far more expensive than the smaller microscope lenses; the reason being that it is exceedingly difficult to obtain a perfect specimen of large size. The difficulty arises not only in the actual manufacture of the glass but in the subsequent operations of cutting, grinding and polishing when fractures are very liable to occur.
From this brief account of the manufacture of optical glass it is clear that a good lens is always worth a high price. Its components must be of the purest quality obtainable, the process of manufacture requires highly skilled labour and it is laborious and exacting work. Constant attention is needed from start to finish and much of the glass is never sufficiently good to pass the rigorous tests which it must undergo. Lastly, in the final preparations of the completed lens, mishaps are frequent. Added to all these trials of the lens maker is the one outstanding fact that the process can never be hurried at any stage, the efficient annealing of optical glass is one of the most important stages in its manufacture. A very full account of the manufacture of optical glass is given in the Encyclopædia Britannica, whence much of our information in this chapter is derived.