The value of a lens depends upon the power of refraction possessed by the material of which the lens is made. The most highly refracting medium is that in which the velocity of the propagation of light is least. The velocity of propagation is diminished by an increase in the density of the material through which the light is passing. The density of a glass varies according to its compo sition. The higher the atomic weight of the metals it contains, the greater is its density, and the higher its refractive index. Many experiments have been made to produce a glass of the greatest density without diminishing its transparency and durability. Faraday has suggested the use for optical purposes of silicated borate of lead, and of silicated triborate of lead. To both of these compounds, want of durability may be urged as an objection, although they possess density in a marked degree. Maez and Clemandot have introduced born-silicate of zinc. This glass has a very pleasing white appearance. It possesses a greater relative density than lead-glass, and can be produced in a condition suited for optical purposes, without being subjected to constant agitation. Lamy's thallium-glass, made from a mixture of silica, red-lead, and carbonate or sulphate of thallium, excels zinc-glass in density, and is homogeneous. Increased density is practically obtained by augmenting the proportion of lead in flint-glass.
Manufacture.--It will be readily understood that, if the density of a lens is not equal throughout, an effect of aberration is produced, and the lens is useless for optical purposes. Molten glass, especially if it contains lead, has a tendency to arrange itself in the crucible according to its density ; the top layers have been found to vary in density between and ; the bottom layers, between 3.85 and 4.75. In order to ensure uniform density and homogeneity, it is essential that the molten mass should be constantly agitated. The materials are melted in covered crucibles, and agitated by means of a hollow fire-clay cylinder, into which an iron bar can be inserted. The cylinder is raised to a white heat in an auxiliary furnace, and introduced into the liquid glass, supported by suitable tackle. The iron bar is inserted, and stirring commences. The iron bar is removed after each stirring, but the fire-clay cylinder is allowed to remain in the crucible. Stirring commences directly the raw materials are thoroughly melted, and is repeated at regular intervals until the glass is ready to be worked. When large lenses are required, and the whole contents of the crucible are to be devoted to the production of one or two lenses, the stirring is continued whilst the furnace is gradually cooled, and so long as the glass is sufficiently mobile to allow of the motion of the stirring-rod, and of the removal of the fire-clay cylinder. For the production of large lenses,
small furnaces, containing only one crucible each, are used, and, so soon as fusion is completed, the fire is allowed slowly to die out, so that the glass contained in the crucible cools with the furnace, and becomes annealed.
When the glass is considered to be sufficiently annealed, the crucible is drawn out of the furnace, and is broken away from the mass of glass contained in it. The glass is now cut, by means of sand, wire, and water, into horizontal slices, which are carefully examined, and re-cut, so as to eliminate defects. Slices thus obtained are placed upon iron moulds, and inserted in a suitable furnace. As the heat of the furnace is increased, the glass adapts itself to the shape of the mould, and obtains from it the rough outline of its future form. After it has been cooled and annealed, the glass is ground, smoothed, and polished. For the manufacture of smaller lenses, the glass in the crucible is stirred and cooled until it has attained a viscous condition. It is now removed, by means of large ladles, into suitable moulds, re-heated, and annealed. It has been suggested for the manu facture of large lenses to pour glass directly from the crucible into moulds or collars, so as to avoid the necessity of putting out the furnace and breaking the crucible. The glass would be immediately moved with the collar or mould into an annealing-kiln. A mass of optical glass may also be ladled from the crucible, and, having been attached to the end of a blow-pipe, be manipulated in the same manner as sheet-glass. The result is a cylinder, usually thicker in substance than ordinary sheet glass, which is annealed, split longitudinally, flattened, and re-annealed. The plate of optical glass thus formed may be cut, ground, and polished, as required.
Coloured light falls upon a transparent body, the body appears colourless, if all the vibrations are transmitted in the proportion in which they exist in the spectrum. If some of the vibrations are checked or extinguished, the emergent light will be of the colour produced by the coexistence of the unchecked vibrations. Certain metals, when in combination with glass, have the power of checking certain vibrations ; some exert a more powerful action than others, and only transmit the least refrangible vibrations. An increased proportion of the metal in the glass, or an increased thickness of the glass, produces the same effect as is gained in the process of sifting by diminishing the mesh of the sieve, or by repeating the operation. If two slips of a glass contain ing iron and cobalt in proper proportions be examined, they will be found separately to transmit a green effect, but when placed together. so as to double the thickness, they will transmit a red colour.