used as the flint, and one containing lead may be employed as the crown, in some optical combinations of lenses. For practi- cally all the optical glasses other than the old flints and crowns the optical industry is indebted in the first instance to the re- searches and manufacture carried out at Jena. Many substances not used before in the production of glass enter into their composition, and it would seem preferable to restrict the expression types to such glasses as have markedly different chemical compositions. It is not necessary to elaborate this point here. It is mentioned only to indicate a distinction between the comparatively few distinct types of optical glasses which have been made and the large number of varieties of them which are needed to provide for the many differences in optical con- stants asked for by opticians.
Investigation and experience have enabled the English manu- facturers of optical glass to go far in satisfying the demands of the manufacturers of optical instruments of all kinds; but there is still room for much experimental research for glasses and transparent media nearer to the ideals present in the minds of designers of optical systems.
With regard to homogeneity, freedom from colour and durability, optical glasses made in England have reached a high level. During the war, in spite of the fact that production had to be so largely increased, the good qualities of the glass generally were not only maintained, but in many instances of glasses somewhat difficult to make a high quality was reached, at least equal to the very best which was available before the war. With the experience gained in recent years improvements of manufacture are possible which point to a greater percentage of yield of high-quality glass than has hitherto been obtained from any melting. The use of more efficient mechanical means for handling pots of glass, the production of pots more highly resistant to the chemical action of molten glass, increase of the durability of some of the less stable glasses, ready production of large homogeneous masses of glass, and the production of new glasses, are the lines along which future progress may be expected, and are the developments indicated during the great activity in the British optical industry owing to the war.
No attempt can be made here to discuss the compositions of the various glasses which have been produced, or to deal in any detail with the range of optical constants now available. Before leaving the subject, however, the relevancy of- the problems connected with optical glass to the production of some other glasses may be mentioned. It is a matter of experience that the numerous researches required for the production of various types of optical glass have a considerable value, not only in arriving at the immediate end in view, but also because the knowledge obtained of the properties of glasses of very varied chemical composition is of the highest importance in pointing the way for designing many glasses for other scientific purposes, and also for certain industrial use. This applies not only to the actual glass, but also to several considerations in respect of furnaces and refractories.
Acknowledgment must be made here of the work organized in England during the war by the Department of Scientific and In- dustrial Research; of work and investigations carried out by the National Physical Laboratory; of the practical investigations by a committee of the Institute of Chemistry; of the work and investiga- tions undertaken by the glass technology department of the univer- sity of Sheffield, and of the guidance and stimulus given by the de- partment of the Ministry of Munitions, which, concerned at first with optical glass only, soon became responsible for supplies of glass and glassware of all descriptions.
Progress in the Use of Machinery. It is probable that between 1910-21 the greatest advance in the economic production of certain types of glassware was in the direction of the introduction of machinery and minor labour-saving devices in substitution of the older hand methods employed in production. Naturally this substitution was only rendered practicable by concurrent improvements in the means for assuring a continuous supply of molten glass in a suitable condition to permit of the machines being run continuously. It will, therefore, be understood that whereas the common practice in the past has been to found the
glass in pots in direct -fired furnaces, there has been a gradual tendency for tanks, some of them being of very large capacity, holding as much as 300 tons of molten glass, to take the place of the older pot furnaces. It is scarcely possible that the pot furnace will entirely disappear from practice, inasmuch as those glasses which are only required in comparatively small quantities or of absolute purity, as in the case of optical glass, certain coloured glasses, and those liable to contamination from furnace gases, will still have to be pot -founded.
The development of machinery in glass manufacture has been by gradual evolution. In general the earlier efforts were directed towards imitating by mechanical means the sequence of the operations performed by the skilled glass worker; and we find, therefore, that skilled labour was not suddenly displaced. The earlier machines were partly automatic or semi-automatic, and required a gatherer, a human link between the furnace and the machine, and also a boy to take and trans- port the finished article from the machine to the annealing lehr. The human links have now been dispensed with in many American factories; neither the raw materials nor the glass is handled at any stage during the progress of manufacture. Conveyors transfer the raw material from the trucks to the storage bin; automatic weighers discharge the requisite quantity of material from the storage bin to a rotary mixer mounted on a trolley; another conveyor transfers the mixed batch to the batch storage bin in close proximity to the charging end of the tank, for the ready release of the batch down a chute at periodic intervals into the tank.
Although machinery has entered so largely into glassware production, there are still some few operations where man has not been displaced. This is more particularly in evidence in the production of many types of chemical glassware produced by the glass-blower with the aid of a blowpipe, the beautiful specimens of cut table-ware, the handiwork of the craftsman skilled in the use of the grinding wheel and polishing pads, and other ornamental ware.
The types of machines may be conveniently divided into the following groups: pressing machines for the production of tumblers, meat and jelly jars, bull's-eye lenses, tableware and pavement lights; press and blow machines for all types of bottles, and many kinds of food containers; blow machines for electric lamp bulbs, lamp chimneys and similar articles; rolling machines for plate glass, figured and ribbed glass and reinforced sheet for sheet and window glass, and for drawing tubes and rods.
In addition to the glass-forming machines there are many other types for miscellaneous purposes, including cracking-off machines for severing the fashioned article from the waste glass, employing multiple fine jets of flame which impinge on the line of severance. This line is usually started by a short diamond cut at the predetermined point. Calibrating machines for accurately dividing measuring devices such as thermometers, burettes, pipettes and cylinders; grinding and polishing machines for preparing and finishing the surface of plate glass; machines for forming the stoppers of bottles and for grinding the seating in the neck of the bottle; flowing devices and feeding machines, to take the place of the gatherer and his operation of withdrawing from the pot or tank, by means of a gathering-iron, a sufficient quantity of molten glass to make the article required.
It will be readily appreciated that in common with certain other industries the development of glass manufacture has made remarkable strides on the engineering side. The advance is the more marked, inasmuch as the progress made in other essen- tials has not beeji commensurate with mechanical progress.
Press Machines. No very marked advance has been made in recent years in this type of machine so far as principle of operation is concerned, but there has been constant improvement in detail. It will be appreciated that a very limited number of types of article can be made with a solid mould. Only such as haye both an internal and external taper, the diameter being reduced in the direction of the movement of the plunger, are suitable. In all other cases where the ware has external shoulders or ornament, the mould must be hinged. In the semi-automatic and fully automatic machines the movements of the plunger and mould are operated by compressed
