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causes striae even after very careful mixture (71. 72), and is
therefore equally to be feared ; and whatever other variations
may have been adopted, I have always found it important to
apply a careful concluding stirring. The following may be
considered as the order of an experiment. If the spongy pla-
tinum has not been introduced into the tray with the rough glass,
then about the sixth hour after lighting the fire it is added in
the manner already directed (85), and the glass well stirred (76).
At about the twelfth hour the stirrings are recommenced, for
the purpose of making the mixture perfect, and are repeated
every 20 or 30 minutes, according to the fusibility of the glass
and the state of the heat (60), for eight or nine times. The glass is

272 On the Manufacture of Optical Glass. [1829.

then allowed to remain at rest for six or eight hours, that bubbles
may ascend and be dissipated, after which it is well stirred
twice or thrice more with particular attention, that, if possible,
no air may be introduced, being thus finally mixed for the last

89. The concluding mixture is peculiar, in that it has to be
continued until the glass is so cold and thick that no ascending
and descending currents can be formed in it ; after which the
temperature is not again to be allowed to rise ; hence the opera-
tion requires certain preliminary arrangements. The first point
necessary is to clear out a considerable quantity of slag from the
flue furnace, or that part beneath the chamber (47). This slag
results from the fused ashes of all the coke which has been
consumed there, with other portions that have passed on from
the coal fire. It is to be drawn on to the bars of the furnace
by a fire-rake which will pass into the passages beneath the
chamber. If not taken out in its fused state, it would be im-
possible afterwards to remove it without risk of great injury to
the furnace. At the same time that the slag is removed, all the
coke is likewise to be withdrawn. All the fuel in the fire-bars
is also to be brought out of the furnace ; and if the bars are
embarrassed with clinkers, they are to be loosened. These
things being done quickly and quietly, and the furnace apertures
closed, a few moments are to be allowed for the little dust that
may have been agitated to settle, and then the chamber is to
be opened and the glass stirred. The heat will have fallen
very little during the preceding operations, and the glass may be
well mixed ; but with this precaution, that when once the stirrer
is beneath the surface, it should not again be taken out until
the conclusion. By opening the feed-hole or the ash-pit, air
may now be allowed freely to enter the furnace, and will rapidly
lower its temperature, especially at such parts as the bottom of
the pan, which are thin and at this moment exposed to the
atmosphere on both surfaces. The temperature of the glass
will fall in a corresponding degree, and the stirring being all
this while continued, though more slowly if convenient, the
substance will gradually thicken, until at last motion will
endanger its being pushed out of the tray, and then the stirrer
is to be carefully withdrawn. No currents in the glass need
be feared, for the temperature cannot now rise higher. But a

1839.] On the Manufacture of Optical Glass. 273

single cover being put over the tray, and the outer orifice of
the air-tube closed by a good cork, the whole may be left a
few minutes to cool still further for perfect security, until, the
glass being supposed to have arrived at the state of a thick
paste, the annealing should commence. Then the ash-pit, the
fire-place, and all the other apertures to the furnace are to be
closed ; the second glass cover put into its place ; the chamber
shut up by its iron and tile covers ; a layer of bricks arranged
close together over the whole upper surface of the chamber
and furnace ; the clamper of the flue closed to prevent air
passing through the fire-place, and the whole left to cool gra-
dually for several days.

90. The interval between the common temperature and that
at which the glass begins to lose solidity and acquire softness,
is so much less, with this variety than with flint glass, that it is
probable a much shorter period of time is required for its perfect
annealing than for the latter. That no failure might occur in
this point, however, four days and nights have been allowed for
the annealing of the large plates. If everything were left as
just described, the contents of the chamber would be warm on
the sixth or even the seventh day, so gradually do the arrange-
ments allow it to cool ; but on the morning or the evening of the
third day, according to circumstances, the damper in the flue is
withdrawn a very little to allow the passage of a small quantity
of air, and by this means the cooling facilitated and regulated.

91. When the furnace and its contents are cold, the chamber
is opened : if the experiment has been well conducted, every-
thing will be found loose, and unaltered in disposition from
what they were when first arranged. The earthenware supports
are to be removed, and the tray taken out. After examining
the glass itself, the exterior of the tray should be carefully
observed, whether there be any appearance of leakages either
through imperceptible holes or at the corners ; and such places
as can be rectified by a patch should be noted in reference to
the future use of the platinum.

92. An operation which, to be successful, requires much care,
is then to be performed ; namely, the separation of the platinum
from the glass. The tray should be placed on clean smooth
paper upon a cloth. The corners are one by one to be opened
by a blunt smooth knife, or some softer instrument, from the
side towards which they were folded ; and being then carefully

274 On the Manufacture of Optical Glass. [1829.

pulled outwards by their extremities, will usually open, so that
the platinum becomes single again. Then proceeding from
corner to corner, the platinum will peel or strip easily from the
sides of the glass, and will remain adhering by the bottom
only. From time to time, as fragments of glass are formed, they
should be blown away or otherwise removed, that they may not
cut the metal. If now the glass be placed a little over the edge
of the table and firmly held, the platinum may gradually be
separated from the bottom in the same manner as from the
sides, and the glass and the metal finally divided from each other
without any injury to the former, and very little to the latter.

93. Immediately upon the separation of the platinum, and
before it can receive any mechanical injury beyond what it
was impossible to avoid, it is to be put into a pickle consisting
of nitric acid and water, and left there for several days. The
dilute acid acts upon the adhering glass, dissolving and loosening
it, and the plate is thus rendered fit for future operations (41).
The stirrers also, when no longer required in an experiment,
should be taken from their iron handles and put into the same
pickling liquor. In this way the platinum is perfectly cleaned,
and being afterwards washed carefully in pure water and
ignited, is again ready for use.

94. Such is the nature of the process as practised at present,
by which plates of heavy optical glass seven inches square and
eight pounds in weight have been prepared. I am encouraged
to believe that it will admit of improvement, perhaps even to
the full extent of our desires ; but it will require time and
patience to effect it. As I have before said, we are in the
course of our experiments only ; and up to the last have seen
reason to vary the arrangements, and still intend to make altera-
tions. Everything agrees to convince me that the size of the
plate is not a circumstance involving any additional difficulty ;
but that, on the contrary, it will probably be safer to make
a large than a small experiment. We can at pleasure obtain a
glass perfectly free from striae^ unexceptionable in hardness,
and with less colour than crown glass ; but it is the simul-
taneous absence of all striae and bubbles, with at the same
time that degree of hardness and colour which will render the
glass fit for optical purposes, that I am aiming at, and that I
trust shortly to obtain.

95. As soon as the plates of glass are removed from the

1829.] On the Manufacture of Optical Glass. 275

platinum and briefly examined, they are sent to Mr. Dollond,
who then enters upon the discharge of his particular duties
in the Committee, by cutting, examining, and even working
them into telescopes. It is not, however, my place to detail
this gentleman's exertions (as a member of the Glass Sub-com-
mittee) in the cause of science. They will, I trust, appear in
due season ; and I hope that the want of perfect success on my
part will not long be a cause of delay.

2. General qualities of the heavy Optical Glasses.

93. A great variety of glasses have been formed by the use
of different proportions of ingredients. They vary importantly
from each other, though by no means to the extent of the
difference existing between any of them and flint glass. The
specific gravity rises very high in borate of lead, consisting of
single proportions, i. e. nearly 24 by weight of boracic acid and
112 of oxide of lead ; it is often as high as 6*39 or 6'4, being
double that of some specimens of flint glass. In silicated borate
of lead, which, in addition to the former quantities, contains
16 parts, or a proportional of silica, it is about 5*44. As the
proportion of oxide of lead diminishes, so also does the specific
gravity lessen, and it is in some of the specimens as low as 4*2 ;
still permitting by the proportions present such fusibility and
other qualities as consist with the process described. The
specific gravity of Guinand's heavy flint glass is about 3'616 ;
that of a specimen of ordinary flint glass 3*290; that of plate
glass 2-5257 ; and that of crown glass 2*5448.

97. The refractive and dispersive powers of the glasses
increase with their specific gravity, as was to be expected.
The powers of two of them, namely, borate of lead and silicated
borate of lead, consisting always, if not otherwise expressed,
of.single proportionals, have been ascertained by Mr. Herschel,
and are as follows :

Sil. Bor.
Bor. Lead. Lead.

Angle of glass prism 29 6' . . 30 26'

Refractive index for extreme red rays /L6 = 2'0430 . . 1*8521
Refractive index for maximum yellow . ^=2-0652 . . 1*8735
Refractive index for extreme violet : : . /4 = 2*1223 . . 1-9135

Dispersive index = -= (K)740 . , 0-0703

276 On the Manufacture of Optical Glass. [1829.

These intense powers upon light are not accompanied by
any circumstance rendering the glass optically unfit for the
compensation of the dispersive powers of crown or plate glass.

Three object-glasses have been constructed for the express
purpose of ascertaining this point ; and all of them tend to
demonstrate that the compensation or correction may be
effected with equal if not greater facility than with flint glass.

98. One important circumstance connected with the appli-
cation of these glasses to the purposes for which they are
designed, is their colour. The great power they have of
developing strong tints from metallic impurities, has been
already described and illustrated (22, 23), and creates a diffi-
culty in the way of obtaining them unobjectionably free from
colour. The usual colour is more or less of yellow, and is
perhaps almost altogether, if not quite, dependent upon the
presence of a little iron. Like many of those dependent upon
mineral substances, it is very much heightened by elevation,
and lessened by diminution of temperature. It is rapidly and
permanently diminished by increasing the proportions either
of the silica or the boracic acid. The silicated borate of lead
has latterly been obtained of such faint tint by the precautions,
relative to impurities, already described, that when 9 inches
in thickness, white paper looked at through it in open day-
light resembled in appearance and depth of tint the surface of
a lemon. Glass consisting of 1 proportional=112 oxide of
lead, 1 proportional=16 silica, and 1J proportional = 36 boracic
acid, when 7 inches in thickness and examined in the same
manner, did not give a colour surpassing that of pale roll
sulphur. The triborate-of-lead glass is almost as colourless
as good flint glass, but might perhaps be found objectionable
on other accounts.

99. As there is a certain quantity of light intercepted by
glass which is altogether dependent upon and in proportion
to its colour, it is evident that this property of the heavy
glasses must be considered in relation to their use in tele-
scopes ; but there appears no reason for supposing they will
ultimately prove inapplicable on this account. The colour of
the glass already obtained is far less in depth than that of the
crown glass constantly used in the construction of telescopes,
which yet intercepts by its colour no important quantity of

1829.'] On the Manufacture of Optical Glass. 277

light; and if two plates 8 or 10 inches long, one of the yellow
heavy glass and the other of crown glass, be looked through
edgeways, it will be seen in a moment that the crown glass
intercepts by far the most light. The colour of the glass is of
no consequence, otherwise than as causing a loss of light from
interception ; for the tinge which is cast over objects looked at
through a telescope constructed with it is scarcely perceptible
to the most acute eye, and quite unimportant. When to these
circumstances is added the reasonable expectation entertained
of removing a large proportion of the little remaining colour
by the use of purified silica (21), it need not be anticipated that
experience will prove the glass faulty in this respect.

100. There is one very important action of the glass upon
light, however, which may perhaps interfere more with its
application, in telescopes at least, than any other, . e. its
reflective power. This is very strong in all the heavy glasses,
far stronger than in flint, and exceedingly surpassing the
similar power of crown glass. It is in proportion, as might
have been expected, to the refractive power and the density
of the glasses, all these properties increasing with the oxide
of lead. The loss of light occasioned by the reflexion from
the two surfaces of a plate through which a ray is passed,
appears to me to be greater than from the united action of
both colour and bubbles in a piece of glass 7 inches thick.

I endeavoured to ascertain the comparative quantities of
light reflected by these heavy and other glasses, in some pho-
tometrical experiments made upon the principle of similar
shadows, measuring only the reflexion from the first surface
of the different glasses, that from the second surface being
destroyed. The ray was made incident in all the cases at an
angle of 45. It was obtained from a small single-wicked
lamp, a ; and when reflected, its intensity was measured by the
distance of a similar lamp, b t whose direct light cast the com-
parative shadow. The uniformity of the two lights, or at least
of their relation to each other, was established by trials before
and after the experiments with the reflecting surfaces, and
each surface was tried two or three times, at intervals, and in
a mixed manner; so that no anticipation of the result could
in any case bias the mind. The following Table shows the
results, small decimals being neglected :

278 On the Manufacture of Optical Glass. [1829.


Light a direct 10-70 ... 1 ... 1

Light a reflected by glass 5 ... 36-75 . . . 11-80 . - ~

1 ... 40-69 . . . 14-46 V ~
4 ... 43-46 . . . 16-50 . . . ^
9 ... 47-31 . . . 19-56 . . . ^ 5

6 ... 50-31 . . . 22-12 . . . ^

7 ... 51-63 . . . 23-29 . . . ^
3 ... 52-69 . . . 24-26 . . .

8 ... 54-33 . . . 25-80 . . . ^

2 ... 54-56 . . . 26-02 . . . ~-

The first column refers to the glasses below ; the second
gives the distance of the measuring flame b\ the third, the
preceding numbers squared and reduced to the direct light as
unity; and the fourth, consequently, the proportion of the
light a reflected by the first surface of each glass. No. 5 was
glass consisting of 1 proportional of oxide of lead, ^ a propor-
tional of silica, and l proportional boracic acid. No. 1 was
composed of 1 oxide of lead, 1 silica, and 1^ boracic acid.
No. 4, of 1 oxide of lead, 1J silica, and 1J boracic acid.
No. 9 was flint glass; No. 6, 7 and 3, different pieces of
crown glass ; and No. 8 and 2, different pieces of plate glass.
1, 3, 5, 6 and 7, were natural surfaces ; 2, 4, 8 and 9, polished

The deficiency of light resulting from the increased re-
flecting power, though considerable, may easily be compensated
for by slightly increasing the area of the plate ; and the power
of obtaining plates of any size is professed to be given by the
general process : but whether that expedient involves any
other objections, it will be for the optician to determine.

101. In hardness, these glasses differ from each other as
much as in any other quality, and indeed more. The borate
of lead is very soft; the biborate of lead is harder, and the
triborate equal to flint glass in hardness, The silicated borate
of lead is softer than flint glass ; but the glass consisting of 1
proportional oxide of lead, 1 of silica, and 1^ proportional of
boracic acid, is as hard as ordinary flint glass, at the same

1829.] On the Manufacture of Optical Glass. 279

time that it has that degree of fusibility, colour, and other
properties, which makes it a very promising variety.

102. The hardness increases with the diminution of the
oxide of lead ; but the fusibility diminishes in the same pro-
portion ; and this is a property which it is essential to preserve
to a certain degree for the removal of striae and bubbles. The
borate of lead is so fusible as to soften and lose its form under
the surface of boiling oil. The silicated borate, and the glass
consisting of the proportions above mentioned, are quite fusible
enough to allow of the processes necessary for the removal of
striae and bubbles.

103. The fusibility of these glasses, and of glass generally,
must not be confounded with their relative tendency to soften
by elevation of temperature. It is not 'that glass which softens
first, that becomes most fluid at a certain given high tempera-
ture ; for glasses, like other substances, vary in their readiness
to pass into the fluid state. Hence it has often occurred
amongst the variety of compositions tried for glasses, that
when the resulting substances have been placed side by side
on platinum foil, and heated, that which first softened did not
when heated highly become so fluid as some other specimens
that longer resisted the first impression of heat. It has, how-
ever, always been found that those glasses which when sub-
jected to a rising temperature, most slowly passed from the
solid to the fluid state, were also those which when subjected
to long annealing processes, were least liable to assume a
crystalline structure ; and thus very useful indications of the
probable qualities of compounds under investigation were often

104. A most important consideration relative to the applica-
tion of these glasses to the construction of telescopes, is their
liability to change and injury by the action of substances
usually occurring in an ordinary atmosphere. When the value
of a good object-glass is considered, frequently amounting to
many hundred pounds, this point will be thought of no little
consequence ; and when it is known that even flint and plate
glass are frequently injured in this way, a little anxiety for the
capability of resistance in the heavy glasses may readily be
allowed, since they contain so much less of the substance
(silica) which confers the power of resistance, and so much

280 On the Manufacture of Optical Glass. [1829.

more of that (oxide of lead) which is considered as the vul-
nerable part, than does either of the former kinds.

105. The superficial changes of glass which interfere with
its optical uses are of two kinds. The one is shown hy a tarnish
upon the surface, which when strong is iridescent. It is quickly
produced hy the intentional presence of sulphuretted hydrogen,
which acting upon the oxide of lead present, reduces it, and
forms a sulphuret of lead. It takes place only with flint glass,
and is in every case produced either by sulphuretted hydrogen
or other sulphuretted vapours. In plate glass the change is of
another kind, and is shown by the appearance of minute vege-
tations or crystallizations, which spread, obstructing the light
wherever they occur. Mr. Dollond, who has shown me cases
of both kinds of injury in flint and plate glass, is inclined to
believe that the latter has, during his long experience, proved
most injurious.

10G. From the commencement of the experiments it was
expected that these heavy optical glasses would tarnish more
than flint glass ; but as specimens of borate of lead and other
dense compounds of that metal had been retained in an ordinary
atmosphere, without any particular precautions, for long periods
of time, yet without tarnishing, there was encouragement to
continue the investigations ; and though when specimens were
put into atmospheres purposely contaminated with sulphuretted
hydrogen, they tarnished quickly, and much more than any
flint glass, yet it did not follow that they would of necessity
tarnish in the telescope ; especially as, being (from the con-
struction of the achromatic object-glass) enclosed by the tube
and the crown or plate glass lens, they would be considerably
protected, and at the same time would admit of the intentional
application of extraneous chemical protectors.

107. The kind of protection which occurs to the mind is the
application of such substances to the interior of the tube as,
having a strong attraction for sulphuretted vapours, should
continually retain the atmosphere within free from their pre-
sence. Carbonate of lead, precipitated borate of lead or finely-
ground litharge, mixed with the pigment which is usually applied
to blacken the inside of the telescope that all extraneous light
may be absorbed, will probably effect this purpose completely.

108. A very curious and important influence of alkali in facili-

1829.] On the Manufacture of Optical Glass. 281

tating the tarnish of glasses containing oxide of lead, was
discovered during the course of these investigations ; and
when the quantity of lead in flint glass is increased but a little
beyond the ordinary proportions, its effect is powerfully mani-
fested. Ordinary flint glass consists of 33*28 oxide of lead,
51*93 silica, and 13*77 potassa; the rest of the substances
present, being in very small quantity, may be disregarded,
Here the oxide of lead is 33*28 hundredths of the whole ; and
if it be only a little increased, for the purpose of giving greater
dispersive power, the glass is liable to tarnish in an ordinary
town atmosphere. Such is the case with a specimen of
Guinand's glass, which I have analyzed, and which contains
4-3*05 oxide of lead, 44*3 silica, and 11*75 potassa. But pro-
vided the alkali be away, the quantity of oxide of lead may be
enormously increased ; and a glass containing 64 per cent, of
oxide of lead, in combination with 36 per cent, of silica, has
not tarnished by an exposure for eighteen months on the same
shelves with flint glasses that have tarnished. The following
case willpoint out the effect still more strongly : A combination
of equal weights of silica and oxide of lead was formed, and
the compound has shown no tendency to tarnish in an ordinary
atmosphere since February 1828. Eight parts of this was
fused with as much pearlash as was equivalent to 1 part of
potassa, and a glass was formed which has since become much
tarnished. But other 8 parts being fused with 3 parts more
of oxide of lead, so as almost to double the proportion of the
latter, gave a glass without alkali, which does not yet exhibit
the slightest trace of tarnish.

109. Hence the reason why the absence of alkali has been
earnestly insisted upon in the preparation of the ingredients
for the heavy optical glasses (18. 24). Hence the reason also
why heavy flint glass, as already mentioned, has tarnished
equally with some of the heavier glasses, though containing so
much less lead, and of such inferior specific gravity. This
influence of alkali is associated with, and perhaps directly
referable to, another circumstance affecting the liability of

Online LibraryMichael FaradayExperimental researches in chemistry and physics → online text (page 27 of 49)