+--------+---------------+--------+------+--------+-----------------------------------------+
| | | | | | 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.
II. BLOWN GLASS. (A) _Table-ware and Vases._--The varieties of glass
used for the manufacture of table-ware and vases are the potash-lead
glass, the soda-lime glass and the potash-lime glass. These glasses may
be colourless or coloured. Venetian glass is a soda-lime glass; Bohemian
glass is a potash-lime glass. The potash-lead glass, which was first
used on a commercial scale in England for the manufacture of table-ware,
and which is known as "flint" glass or "crystal," is also largely used
in France, Germany and the United States. Table II. shows the typical
composition of these glasses.