CHAPTER XXI
HOW THE GLASS USED IN MICROSCOPES IS MADE
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.