Chapter I: OPTICAL GLASS.--As regards both mode of production and essential

properties optical glass differs widely from all other varieties. These differences arise primarily from the fact that glass for optical uses is required in comparatively large and thick pieces, while for most other purposes glass is used in the form of comparatively thin sheets; when, therefore, as a consequence of Dollond's invention of achromatic telescope objectives in 1757, a demand first arose for optical glass, the industry was unable to furnish suitable material. Flint glass particularly, which appeared quite satisfactory when viewed in small pieces, was found to be so far from homogeneous as to be useless for lens construction. The first step towards overcoming this vital defect in optical glass was taken by P. L. Guinand, towards the end of the 18th century, by introducing the process of stirring the molten glass by means of a cylinder of fireclay. Guinand was induced to migrate from his home in Switzerland to Bavaria, where he worked at the production of homogeneous flint glass, first with Joseph von Utzschneider and then with J. Fraunhofer; the latter ultimately attained considerable success and produced telescope disks up to 28 centimetres (11 in.) diameter. Fraunhofer further initiated the specification of refraction and dispersion in terms of certain lines of the spectrum, and even attempted an investigation of the effect of chemical composition on the relative dispersion produced by glasses in different parts of the spectrum. Guinand's process was further developed in France by Guinand's sons and subsequently by Bontemps and E. Feil. In 1848 Bontemps was obliged to leave France for political reasons and came to England, where he initiated the optical glass manufacture at Chance's glass works near Birmingham, and this firm ultimately attained a considerable reputation in the production of optical glass, especially of large disks for telescope objectives. Efforts at improving optical glass had, however, not been confined to the descendants and successors of Guinand and Fraunhofer. In 1824 the Royal Astronomical Society of London appointed a committee on the subject, the experimental work being carried out by Faraday. Faraday independently recognized the necessity for mechanical agitation of the molten glass in order to ensure homogeneity, and to facilitate his manipulations he worked with dense lead borate glasses which are very fusible, but have proved too unstable for ordinary optical purposes. Later Maes of Clichy (France) exhibited some "zinc crown" glass in small plates of optical quality at the London Exhibition of 1851; and another French glass-maker, Lamy, produced a dense thallium glass in 1867. In 1834 W. V. Harcourt began experiments in glass-making, in which he was subsequently joined by G. G. Stokes. Their object was to pursue the inquiry begun by Fraunhofer as to the effect of chemical composition on the distribution of dispersion. The specific effect of boric acid in this respect was correctly ascertained by Stokes and Harcourt, but they mistook the effect of titanic acid. J. Hopkinson, working at Chance's glass works, subsequently made an attempt to produce a titanium silicate glass, but nothing further resulted.

The next and most important forward step in the progress of optical glass manufacture was initiated by Ernst Abbe and carried out jointly by him and O. Schott at Jena in Germany. Aided by grants from the Prussian government, these workers systematically investigated the effect of introducing a large number of different chemical substances (oxides) into vitreous fluxes. As a result a whole series of glasses of novel composition and optical properties were produced. A certain number of the most promising of these, from the purely optical point of view, had unfortunately to be abandoned for practical use owing to their chemical instability, and the problem of Fraunhofer, viz. the production of pairs of glasses of widely differing refraction and dispersion, but having a similar distribution of dispersion in the various regions of the spectrum, was not in the first instance solved. On the other hand, while in the older crown and flint glasses the relation between refraction and dispersion had been practically fixed, dispersion and refraction increasing regularly with the density of the glass, in some of the new glasses introduced by Abbe and Schott this relation is altered and a relatively low refractive index is accompanied by a relatively high dispersion, while in others a high refractive index is associated with low dispersive power.

The initiative of Abbe and Schott, which was greatly aided by the resources for scientific investigation available at the Physikalische Reichsanstalt (Imperial Physical Laboratory), led to such important developments that similar work was undertaken in France by the firm of Mantois, the successors of Feil, and somewhat later by Chance in England. The manufacture of the new varieties of glass, originally known as "Jena" glasses, is now carried out extensively and with a considerable degree of commercial success in France, and also to a less extent in England, but none of the other makers of optical glass has as yet contributed to the progress of the industry to anything like the same extent as the Jena firm.

The older optical glasses, now generally known as the "ordinary" crown and flint glasses, are all of the nature of pure silicates, the basic constituents being, in the case of crown glasses, lime and soda or lime and potash, or a mixture of both, and in the case of flint glasses, lead and either (or both) soda and potash. With the exception of the heavier flint (lead) glasses, these can be produced so as to be free both from noticeable colour and from such defects as bubbles, opaque inclusions or "striae," but extreme care in the choice of all the raw materials and in all the manipulations is required to ensure this result. Further, these glasses, when made from properly proportioned materials, possess a very considerable degree of chemical stability, which is amply sufficient for most optical purposes. The newer glasses, on the other hand, contain a much wider variety of chemical constituents, the most important being the oxides of barium, magnesium, aluminium and zinc, used either with or without the addition of the bases already named in reference to the older glasses, and--among acid bodies--boric anhydride (B2O3) which replaces the silica of the older glasses to a varying extent. It must be admitted that, by the aid of certain of these new constituents, glasses can be produced which, as regards purity of colour, freedom from defects and chemical stability are equal or even superior to the best of the "ordinary" glasses, but it is a remarkable fact that when this is the case the optical properties of the new glass do not fall very widely outside the limits set by the older glasses. On the other hand, the more extreme the optical properties of these new glasses, i.e. the further they depart from the ratio of refractive index to dispersive power found in the older glasses, the greater the difficulty found in obtaining them of either sufficient purity or stability to be of practical use. It is, in fact, admitted that some of the glasses, most useful optically, the dense barium crown glasses, which are so widely used in modern photographic lenses, cannot be produced entirely free either from noticeable colour or from numerous small bubbles, while the chemical nature of these glasses is so sensitive that considerable care is required to protect the surfaces of lenses made from them if serious tarnishing is to be avoided. In practice, however, it is not found that the presence either of a decidedly greenish-yellow colour or of numerous small bubbles interferes at all seriously with the successful use of the lenses for the majority of purposes, so that it is preferable to sacrifice the perfection of the glass in order to secure valuable optical properties.

It is a further striking fact, not unconnected with those just enumerated, that the extreme range of optical properties covered even by the relatively large number of optical glasses now available is in reality very small. The refractive indices of all glasses at present available lie between 1.46 and 1.90, whereas transparent minerals are known having refractive indices lying considerably outside these limits; at least one of these, fluorite (calcium fluoride), is actually used by opticians in the construction of certain lenses, so that probably progress is to be looked for in a considerable widening of the limits of available optical materials; possibly such progress may lie in the direction of the artificial production of large mineral crystals.

The qualities required in optical glasses have already been partly referred to, but may now be summarized:--

1. _Transparency and Freedom from Colour._--These qualities can be
readily judged by inspection of the glass in pieces of considerable
thickness, and they may be quantitatively measured by means of the
spectro-photometer.

2. _Homogeneity._--The optical desideratum is uniformity of refractive
index and dispersive power throughout the mass of the glass. This is
probably never completely attained, variations in the sixth
significant figure of the refractive index being observed in
different parts of single large blocks of the most perfect glass.
While such minute and gradual variations are harmless for most optical
purposes, sudden variations which generally take the form of striae or
veins are fatal defects in all optical glass. In their coarsest forms
such striae are readily visible to the unaided eye, but finer ones
escape detection unless special means are taken for rendering them
visible; such special means conveniently take the form of an apparatus
for examining the glass in a beam of parallel light, when the striae
scatter the light and appear as either dark or bright lines according
to the position of the eye. Plate glass of the usual quality, which
appears to be perfectly homogeneous when looked at in the ordinary
way, is seen to be a mass of fine striae, when a considerable
thickness is examined in parallel light. Plate glass is, nevertheless,
considerably used for the cheaper forms of lenses, where the
scattering of the light and loss of definition arising from these fine
striae is not readily recognized.

Bubbles and enclosures of opaque matter, although more readily
observed, do not constitute such serious defects; their presence in a
lens, to a moderate extent, does not interfere with its performance
(see above).

3. _Hardness and Chemical Stability._--These properties contribute to
the durability of lenses, and are specially desirable in the outer
members of lens combinations which are likely to be subjected to
frequent handling or are exposed to the weather. As a general rule, to
which, however, there are important exceptions, both these qualities
are found to a greater degree, the lower the refractive index of the
glass. The chemical stability, i.e. the power of resisting the
disintegrating effects of atmospheric moisture and carbonic acid,
depends largely upon the quantity of alkalis contained in the glass
and their proportion to the lead, lime or barium present, the
stability being generally less the higher the proportion of alkali. A
high silica-content tends towards both hardness and chemical
stability, and this can be further increased by the addition of small
proportions of boric acid; in larger quantities, however, the latter
constituent produces the opposite effect.

4. _Absence of Internal Strain._--Internal strain in glass arises from
the unequal contraction of the outer and inner portions of masses of
glass during cooling. Processes of annealing, or very gradual cooling,
are intended to relieve these strains, but such processes are only
completely effective when the cooling, particularly through those
ranges of temperature where the glass is just losing the last traces
of plasticity, is extremely gradual, a rate measured in hours per
degree Centigrade being required. The existence of internal strains in
glass can be readily recognized by examination in polarized light, any
signs of double refraction indicating the existence of strain. If the
glass is very badly annealed, the lenses made from it may fly to
pieces during or after manufacture, but apart from such extreme cases
the optical effects of internal strain are not readily observed except
in large optical apparatus. Very perfectly annealed optical glass is
now, however, readily obtainable.

5. _Refraction and Dispersion._--The purely optical properties of
refraction and dispersion, although of the greatest importance, cannot
be dealt with in any detail here; for an account of the optical
properties required in glasses for various forms of lenses see the
articles LENS and ABERRATION: II. _In Optical Systems_. As typical of
the range of modern optical glasses Table I. is given, which
constituted the list of optical glasses exhibited by Messrs Chance at
the Optical Convention in London in 1905. In this table n is the
refractive index of the glass for sodium light (the D line of the
solar spectrum), while the letters C, F and G' refer to lines in the
hydrogen spectrum by which dispersion is now generally specified. The
symbol [nu] represents the inverse of the dispersive power, its value
being (n_D - 1)/(C - F). The very much longer lists of German and
French firms contain only a few types not represented in this table.

Table I.--_Optical Properties._

+--------+---------------+--------+------+--------+-----------------------------------------+
| | | | | | Partial and Relative |
| | | | | Medium | Partial Dispersions. |
| Factory| | | | Disper-+-------+-----+-------+-----+-------+-----+
| Number.| Name. | n_D. | [nu].| sion. | | C-D | | D-F | | F-G'|
| | | | | C-F. | C-D. | --- | D-F. | --- | F-G'. | --- |
| | | | | | | C-F | | C-F | | C-F.|
+--------+---------------+--------+------+--------+-------+-----+-------+-----+-------+-----+
| C. 644 | Extra Hard | | | | | | | | | |
| | Crown | 1.4959 | 64.4 | .00770 |.00228 |.296 |.00542 |.704 |.00431 |.560 |
| B. 646 | Boro-silicate | | | | | | | | | |
| | Crown | 1.5096 | 63.3 | .00803 |.00236 |.294 |.00562 |.700 |.00446 |.555 |
| A. 605 | Hard Crown | 1.5175 | 60.5 | .00856 |.00252 |.294 |.00604 |.706 |.00484 |.554 |
| C. 577 | Medium Barium | | | | | | | | | |
| | Crown | 1.5738 | 57.9 | .00990 |.00293 |.296 |.00697 |.704 |.00552 |.557 |
| C. 579 | Densest Barium| | | | | | | | | |
| | Crown | 1.6065 | 57.9 | .01046 |.00308 |.294 |.00738 |.705 |.00589 |.563 |
| A. 569 | Soft Crown. | 1.5152 | 56.9 | .00906 |.00264 |.291 |.00642 |.708 |.00517 |.570 |
| B. 563 | Medium Barium | | | | | | | | | |
| | Crown | 1.5660 | 56.3 | .01006 |.00297 |.295 |.00709 |.704 |.00576 |.572 |
| B. 535 | Barium Light | | | | | | | | | |
| | Flint | 1.5452 | 53.5 | .01020 |.00298 |.292 |.00722 |.701 |.00582 |.570 |
| A. 490 | Extra Light | | | | | | | | | |
| | Flint | 1.5316 | 49.0 | .01085 |.00313 |.288 |.00772 |.711 |.00630 |.580 |
| A. 485 | Extra Light | | | | | | | | | |
| | Flint | 1.5333 | 48.5 | .01099 |.00322 |.293 |.00777 |.707 |.00643 |.582 |
| C. 474 | Boro-silicate | | | | | | | | | |
| | Flint | 1.5623 | 47.4 | .01187 |.00343 |.289 |.00844 |.711 |.00693 |.584 |
| B. 466 | Barium Light | | | | | | | | | |
| | Flint | 1.5833 | 46.6 | .01251 |.00362 |.288 |.00889 |.711 |.00721 |.576 |
| B. 458 | Soda Flint | 1.5482 | 45.8 | .01195 |.00343 |.287 |.00852 |.713 |.00690 |.577 |
| A. 458 | Light Flint | 1.5472 | 45.8 | .01196 |.00348 |.291 |.00848 |.709 |.00707 |.591 |
| A. 432 | Light Flint | 1.5610 | 43.2 | .01299 |.00372 |.287 |.00927 |.713 |.00770 |.593 |
| A. 410 | Light Flint | 1.5760 | 41.0 | .01404 |.00402 |.286 |.01002 |.713 |.00840 |.598 |
| B. 407 | Light Flint | 1.5787 | 40.7 | .01420 |.00404 |.284 |.01016 |.715 |.00840 |.591 |
| A. 370 | Dense Flint. | 1.6118 | 36.9 | .01657 |.00470 |.284 |.01187 |.716 |.01004 |.606 |
| A. 361 | Dense Flint. | 1.6214 | 36.1 | .01722 |.00491 |.285 |.01231 |.715 |.01046 |.608 |
| A. 360 | Dense Flint. | 1.6225 | 36.0 | .01729 |.00493 |.286 |.01236 |.715 |.01054 |.609 |
| A. 337 | Extra Dense | | | | | | | | | |
| | Flint | 1.6469 | 33.7 | .01917 |.00541 |.285 |.01376 |.720 |.01170 |.655 |
| A. 299 | Densest Flint | 1.7129 | 29.9 | .02384 |.00670 |.281 |.01714 |.789 |.01661 |.678 |
+--------+---------------+--------+------+--------+-------+-----+-------+-----+-------+-----+

_Manufacture of Optical Glass._--In its earlier stages, the process for the production of optical glass closely resembles that used in the production of any other glass of the highest quality. The raw materials are selected with great care to assure chemical purity, but whereas in most glasses the only impurities to be dreaded are those that are either infusible or produce a colouring effect upon the glass, for optical purposes the admixture of other glass-forming bodies than those which are intended to be present must be avoided on account of their effect in modifying the optical constants of the glass. Constancy of composition of the raw materials and their careful and thorough admixture in constant proportions are therefore essential to the production of the required glasses. The materials are generally used in the form either of oxides (lead, zinc, silica, &c.) or of salts readily decomposed by heat, such as the nitrates or carbonates. Fragments of glass of the same composition as that aimed at are generally incorporated to a limited extent with the mixed raw materials to facilitate their fusion. The crucibles or pots used for the production of optical glass very closely resemble those used in the manufacture of flint glass for other purposes; they are "covered" and the molten materials are thus protected from the action of the furnace gases by the interposition of a wall of fireclay, but as crucibles for optical glass are used for only one fusion and are then broken up, they are not made so thick and heavy as those used in flint-glass making, since the latter remain in the furnace for many weeks. On the other hand, the chemical and physical nature of the fireclays used in the manufacture of such crucibles requires careful attention in order to secure the best results. The furnace used for the production of optical glass is generally constructed to take one crucible only, so that the heat of the furnace may be accurately adjusted to the requirements of the particular glass under treatment. These small furnaces are frequently arranged for direct coal firing, but regenerative gas-fired furnaces are also employed. The empty crucible, having first been gradually dried and heated to a bright red heat in a subsidiary furnace, is taken up by means of massive iron tongs and introduced into the previously heated furnace, the temperature of which is then gradually raised. When a suitable temperature for the fusion of the particular glass in question has been attained, the mixture of raw materials is introduced in comparatively small quantities at a time. In this way the crucible is gradually filled with a mass of molten glass, which is, however, full of bubbles of all sizes. These bubbles arise partly from the air enclosed between the particles of raw materials and partly from the gaseous decomposition products of the materials themselves. In the next stage of the process, the glass is raised to a high temperature in order to render it sufficiently fluid to allow of the complete elimination of these bubbles; the actual temperature required varies with the chemical composition of the glass, a bright red heat sufficing for the most fusible glasses, while with others the utmost capacity of the best furnaces is required to attain the necessary temperature. With these latter glasses there is, of course, considerable risk that the partial fusion and consequent contraction of the fireclay of the crucible may result in its destruction and the entire loss of the glass. The stages of the process so far described generally occupy from 36 to 60 hours, and during this time the constant care and watchfulness of those attending the furnace is required. This is still more the case in the next stage. The examination of small test-pieces of the glass withdrawn from the crucible by means of an iron rod having shown that the molten mass is free from bubbles, the stirring process may be begun, the object of this manipulation being to render the glass as homogeneous as possible and to secure the absence of veins or striae in the product. For this purpose a cylinder of fireclay, provided with a square axial hole at the upper end, is heated in a small subsidiary furnace and is then introduced into the molten glass. Into the square axial hole fits the square end of a hooked iron bar which projects several yards beyond the mouth of the furnace; by means of this bar a workman moves the fireclay cylinder about in the glass with a steady circular sweep. Although the weight of the iron bar is carried by a support, such as an overhead chain or a swivel roller, this operation is very laborious and trying, more especially during the earlier stages when the heat radiated from the open mouth of the crucible is intense. The men who manipulate the stirring bars are therefore changed at short intervals, while the bars themselves have also to be changed at somewhat longer intervals, as they rapidly become oxidized, and accumulated scale would tend to fail off them, thus contaminating the glass below. The stirring process is begun when the glass is perfectly fluid at a temperature little short of the highest attained in its fusion, but as the stirring proceeds the glass is allowed to cool gradually and thus becomes more and more viscous until finally the stirring cylinder can scarcely be moved. When the glass has acquired this degree of consistency it is supposed that no fresh movements can occur within its mass, so that if homogeneity has been attained the glass will preserve it permanently. The stirring is therefore discontinued and the clay cylinder is either left embedded in the glass, or by the exercise of considerable force it may be gradually withdrawn. The crucible with the semi-solid glass which it contains is now allowed to cool considerably in the melting furnace, or it may be removed to another slightly heated furnace. When the glass has cooled so far as to become hard and solid, the furnace is hermetically sealed up and allowed to cool very gradually to the ordinary temperature. If the cooling is very gradual--occupying several weeks--it sometimes happens that the entire contents of a large crucible, weighing perhaps 1000 lb., are found intact as a single mass of glass, but more frequently the mass is found broken up into a number of fragments of various sizes. From the large masses great lenses and mirrors may be produced, while the smaller pieces are used for the production of the disks and slabs of moderate size, in which the optical glass of commerce is usually supplied. In order to allow of the removal of the glass, the cold crucible is broken up and the glass carefully separated from the fragments of fireclay. The pieces of glass are then examined for the detection of the grosser defects, and obviously defective pieces are rejected. As the fractured surfaces of the glass in this condition are unsuitable for delicate examination a good deal of glass that passes this inspection has yet ultimately to be rejected. The next stage in the preparation of the glass is the process of moulding and annealing. Lumps of glass of approximately the right weight are chosen, and are heated to a temperature just sufficient to soften the glass, when the lumps are caused to assume the shape of moulds made of iron or fireclay either by the natural flow of the softened glass under gravity, or by pressure from suitable tools or presses. The glass, now in its approximate form, is placed in a heated chamber where it is allowed to cool very gradually--the minimum time of cooling from a dull red heat being six days, while for "fine annealing" a much longer period is required (see above). At the end of the annealing process the glass issues in the shape of disks or slabs slightly larger than required by the optician in each case. The glass is, however, by no means ready for delivery, since it has yet to be examined with scrupulous care, and all defective pieces must be rejected entirely or at least the defective part must be cut out and the slab remoulded or ground down to a smaller size. For the purpose of rendering this minute examination possible, opposite plane surfaces of the glass are ground approximately flat and polished, the faces to be polished being so chosen as to allow of a view through the greatest possible thickness of glass; thus in slabs the narrow edges are polished.

It will be readily understood from the above account of the process of production that optical glass, relatively to other kinds of glass, is very expensive, the actual price varying from 3s. to 30s. per lb. in small slabs or disks. The price, however, rapidly increases with the total bulk of perfect glass required in one piece, so that large disks of glass suitable for telescope objectives of wide aperture, or blocks for large prisms, become exceedingly costly. The reason for this high cost is to be found partly in the fact that the yield of optically perfect glass even in large and successful meltings rarely exceeds 20% of the total weight of glass melted. Further, all the subsequent processes of cutting, moulding and annealing become increasingly difficult, owing to the greatly increased risk of breakage arising from either external injury or internal strain, as the dimensions of the individual piece of glass increase. Nevertheless, disks of optical glass, both crown and flint, have been produced up to 39 in. in diameter.