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THE BAKERIAN LECTURE. On the Manufacture of Glass for

Optical Purposes-^-.
[Read November 19, December 3 and 10, 1829.]

Introduction.

PERFECT as is the manufacture of glass for all ordinary pur-
poses, and extensive the scale upon which its production is
carried on, yet there is scarcely any artificial substance in
which it is so difficult to unite what is required to satisfy
the wants of science. Its general transparency, hardness,
unchangeable nature and varied refractive and dispersive
powers, render glass a most important agent in the hands of
the philosopher engaged in investigating the nature and pro-
perties of light ; but when he desires to apply it, according to
the laws he has discovered, in the construction of perfect in-
struments, and especially of the achromatic telescope, it is

* Quarterly Journal of Science, xxii. 399, or Ann. Phil. N. S. xx. 401.

t Philosophical Transactions, 1830, p. 1. The use of the glass manufac-
tured as described has since become so important in diamagnetic and magneto-
optical researches, that I deem the paper worthy of insertion at full length in
cthis collection. M. F. 1858.



232 On the Manufacture of Optical Glass. [1829.

found liable to certain imperfections, not essentially existing,
but almost always involved during its preparation, and fatal to
its use. These are so important and so difficult to avoid, that
science is frequently stopped in her progress by them ; a fact
fully proved by the circumstance that Mr. Dollond, one of our
first opticians, has not been able to obtain a disc of flint-glass
four inches and a half in diameter, fit for a telescope, within
the last five years, or a similar disc of five inches in diameter
within the last ten years.

It must be well known to the scientific world, that these
difficulties have induced some persons to labour hard and
earnestly for years together, in hopes of surmounting them.
Guinand was one of these : his means were small, but he de-
serves the more honour for his perseverance and his success.
He commenced the investigation about the year 1784, and died
engaged in it in the year 1823. Fraunhofer laboured hard at
the solution of the same practical problem. He was a man of
profound science, and had all the advantages arising from
extensive means and information, both in himself and others.
He laboured in the glass house, the workshop, and the study,
pursuing without deviation the great object he had in view,
until science was deprived of him also by death. Both these
men, according to the best evidence we can obtain, have pro-
duced and left some perfect glass in large pieces : but whether
it is that the knowledge they acquired was altogether practical
and personal, a matter of minute experience, and not of a nature
to be communicated ; or whether other circumstances were
connected with it, it is certain that the public are not in pos-
session of any instruction, relative to the method of making a
homogeneous glass fit for optical purposes, beyond what was
possessed before their time ; and in this country it seems doubt-
ful whether they ever attained a method of making such glass
with certainty and at pleasure, or have left any satisfactory in-
structions on the subject behind them.

The philosophical deficiencies referred to above, induced the
President and Council of the Royal Society in 1824, to appoint
a Committee for the improvement of glass for optical purposes,
consisting of Fellows of the Royal Society and Members of the
then Board of Longitude. The Government on being applied
to, not only removed the restrictions to experiments on glass,



1829.] On the Manufacture of Optical Glass. 233

occasioned by the Excise laws and regulations, but undertook
to bear all the expenses of furnaces, materials, and labour, as
long as the investigations offered a reasonable hope of success.
In consequence of these facilities, a small glass furnace was
erected in 1825, and many experiments both upon a large and
small scale were made with flint and other glasses. During
their continuance, Messrs. Green and Pellatt gave every in-
struction and assistance in their power, and evinced the most
earnest desire for success. The researches, however, soon
showed themselves to be a work of labour, which, to be suc-
cessful, would require to be pursued unremittingly for a long
period; and on May 5, 1825, a sub-committee was appointed,
to whom the direct superintendence and performance of experi-
ments were entrusted. This committee consisted of Mr. Her-
schel, Mr. Dollond, and myself; but in March 1829 was reduced
to two, by the retirement of Mr. Herschel, who about that
period went to the continent. From the respective pursuits of
the three persons appointed upon this committee it may be
easily gathered, that though all were to do what they could in
everyway for the general good of the cause in which they were
jointly engaged, yet a distinction in the duties of each existed.
It was my business to investigate particularly the chemical part
of the inquiry ; Mr. Dollond was to work and try the glass, and
ascertain practically its good or bad qualities ; whilst Mr. Her-
schel was to examine its physical properties, reason respecting
their influence and utility, and make his competent mind bear
upon every part of the inquiry.

The experimental glass house was erected on a part of the
premises of Messrs. Green and Pellatt, at the Falcon Glass-
works ; whilst my duties as Director of the Laboratory of the
Royal Institution, required my presence almost constantly at
the latter place, nearly three miles from the former. As I
found it impossible under these circumstances to make the
numerous experiments and pay that close attention which ap-
peared essentially necessary to produce any degree of success,
the President and Council of the Royal Society applied to the
President and Managers of the Royal Institution, for leave to
erect on their premises an experimental room, with a furnace,
for the purpose of continuing the investigation. They were
guided in this by the desire which the Royal Institution has



On the Manufacture of Optical Glass. [1829.

always evinced to assist in the advancement of science; and
the readiness with which the application was granted, showed
that no mistaken notion had been formed in this respect. As
a member of both bodies, I felt much anxiety that the investi-
gation should be successful. A room and furnaces were built
at the Royal Institution in September 1827, and an assistant
was engaged, Sergeant Anderson of the Royal Artillery, whose
steady and intelligent care has been of the greatest service to
me in the experiments that have been proceeding constantly
from that time to the present. At first, the inquiry was pur-
sued principally as related to flint and ground glass ; but in
September 1828 it was directed exclusively to the preparation
and perfection of peculiar heavy and fusible glasses, from which
time to the present continual progress has been made.

I have thought it right to give this brief explanatory state-
ment of the manner in which it has happened to become my
duty, on the present occasion, to give an account of what has
been done in the improvement of glass for optical purposes by
the Committee of the Royal Society, working at the Royal In-
stitution. I would willingly have deferred this account until
the inquiry were more complete than at present ; for though
glass has been made, and telescopes manufactured, yet I have
no doubt that much more of improvement will be effected. It
may be said that a long time has elapsed since the experiments
were first instituted ; and that if anything could be done, it
should have been effected in so long a period. But be it re-
membered, that it is not a mere analysis, or even the develop-
ment of philosophical reasoning, that is required : it is the
solution of difficulties, which, as in the cases of Guinand and
Fraunhofer, required many years of a practical life to effect, if
it was ever effected. It is the foundation and development of
a manufacturing process, not in principle only, but through all
the difficulties of practice, until it is competent to give constant
success : and I may be allowed to plead the acknowledged dif-
ficulty and importance of the subject as a reason, both why it
may not yet have obtained perfection, and why it should still
be pursued.

My wish, however, to delay the account of the researches
until I could have carried the experiments further, is overcome
by the conviction that much more time must be expected to



1829.] On the Manufacture of Qirtical Glass. 235

elapse before I shall consider the investigation finished ; by the
consideration that a decided step has been made in the manu-
facture of glass for optical purposes ; and by the feeling that
the Royal Society which instituted, and the Government which
defrays the expenses of the experiments, have a right to an offi-
cial account of the present state of the investigation. Although
much useful information has been obtained respecting flint and
other glasses, yet as that train of research is very imperfect,
uncertain, and will probably be resumed, I shall confine my
present statement altogether to the heavy optical glass already
referred to. It will be impossible for me to describe all that
has been done on this subject ; but I shall endeavour to give
such an account of the glass, and the process by which it is
obtained in a homogeneous state, as shall enable other persons
to do what has been done at the Royal Institution, without in-
curring the laborious prefatory experiments and investigations
which we have had to undertake ; only introducing so much of
the latter, and the principles of the process, as are necessary
to make the descriptions clear to a practical man, and enable
him to avoid those circumstances which might otherwise occa-
sion failure. That the paper may appear long and tedious I
am aware ; but it should be remembered, that it can have no
other utility than as containing efficient instructions to the few
who may desire to manufacture optical glass ; and that to ren-
der whatever of this character it may have, imperfect, for the
sake of giving to it a more abbreviated and popular form, would
have been doing injustice to the objects and motives of those
who have instituted and supported the experiments.

1. Process of Manufacture > $c.

1. The general properties of transparency, hardness, and a
certain degree of refractive and dispersive power, which render
glass so valuable as an optical agent, are easily obtained : but
there is one condition essential in all delicate cases of its appli-
cation, which is not so readily fulfilled ; this is, a perfectly ho-
mogeneous composition and structure. Although every part
of the glass may in itself be as good as possible, yet without
this condition they do not act in uniformity with each other ;
the rays of light are deflected from the course which they ought
to pursue, and the piece of glass becomes useless. The streaks,



236 On the Manufacture of Optical Glass. [1829.

striae, veins or tails, which are seen within glass otherwise per-
fectly good, result from a want of this equality ; they are visible
only because they bend the rays of light which pass through
them from their rectilinear course, and are constituted of a glass
having either a greater or a smaller refractive power than the
neighbouring parts.

2. When these irregularities are so powerful as to render
their effects observable by the naked eye, it may easily be sup-
posed to what an injurious extent their influence must extend
in the construction of telescopes and other instruments of a
similar nature, where these faults are not only magnified many
times, but where the effect is to give an equally magnified erro-
neous representation of the object looked at, when the very
point to be attained is to examine that object with the utmost
accuracy ; and it is accordingly found that these striae are the
most fatal faults of glass intended for optical purposes. Besides
this, not only do the strige themselves occasion harm, but there
is every reason to believe that they rarely occur in glass other-
wise homogeneous. Sometimes, it is true, a grain of sand, in
passing through and at the same time dissolving in glass, will
give a streak of different composition to the rest of the sub-
stance ; and at others, a bubble ascending may lift a line of
heavy or more refractive matter into a lighter and less refrac-
tive portion above. But very often, and especially as glass is
usually manufactured and collected for use, striae are merely
the lines or planes where two different kinds of glass approxi-
mate ; and even if the striae could be covered so as to produce
no bad effect, yet the other parts, not being in every respect
alike, would exert an unequal action on light, and the piece be
therefore improper for the construction of a telescope. Many
a disc, which upon the most careful examination has appeared
perfectly free from striae and quite uniform, has, when worked
into an object-glass, been found incapable of giving a good
image, on account of the existence of irregularities in the mass,
which, though not sudden or strong enough to occasion striae,
still produce a confused effect ; and if this happens with glass
approaching so near to perfection, it happens still more fre-
quently and to a much stronger degree with such as contain
visible irregularities.

3. It must not be imagined that striae, or those fainter differ-



1829.] On the Manufacture of Optical Glass.

ences, are, according to an expression sometimes used, due to
impurity. The glass, either of the streak or of the neighbour-
ing parts, would be equally good for optical purposes were it
all alike. It is the irregularity that constitutes the fault ; and
hence, in this respect, a particular composition is of very little
importance. As glass is always the result of a mixture of ma-
terials having different refractive and dispersive powers, it is
evident that striae must exist at one period during its prepara-
tion ; and the point required is not so much to seek for a differ-
ence of composition, or for those proportions which are found
by analysis to exist in specimens of tried and acknowledged
good glass, as to devise and perfect a process by which the
striae period should be passed over before the glass is finished
and the formation of fresh striae be prevented.

4. Besides these, there are other faults in glass. Sometimes
it is said to be wavy, when it has the appearance of waves
within its mass ; but this is only a variety of that irregularity
which has just been explained as constituting, when in a
stronger degree, streaks and striae. Occasionally appearances
are observed in it, which seem to indicate a peculiar structure
of crystallization, or an irregular tension of its parts : these,
there is every reason to believe, may be avoided by careful an-
nealing. Again : the glass sometimes includes bubbles, which,
when small and numerous, render it what is called seedy.
Bubbles are not usually considered as of much consequence to
the performance of the glass, but objectionable only because of
their appearance when the glass is looked at, rather than when
looked through. They each act like a very powerful but very
small double convex lens of a rare substance in a very dense
medium, or as equally deep double concave lenses of glass
would do in air; they rapidly, therefore, turn the rays im-
pinging on them on one side, and occasion a loss of light, just
as so many opake spots would do. But as even when numerous
their united area may amount to only a very small proportion of
the area of the plate of glass required for a telescope, this loss
of light is usually of but little consequence. In practice, it is
said that no other real evil than such loss of light is dependent
on them.

5. Of all these faults, that of the irregularity constituting
streaks, striae, and waves, is the most difficult to avoid, and the



238 On the Manufacture of Optical Glass. [1829.

most injurious in its effect. It is not an improvement only be-
yond what is ordinarily done in this respect that is required,
but absolute perfection, a homogeneity equal to that of pure
water. In the two kinds of glass required to render a telescope
achromatic, namely, crown or plate glass, and flint glass, it is
the latter which is obtained perfect with the greatest difficulty,
and to which therefore the greatest attention has been paid.
The reason of this will be evident, if the general composition
of the two glasses be taken into account. The required differ-
ence between them in refractive and dispersive power is found
to be at command, by attention to composition ; and it has been
also ascertained, that crown and plate glass answer exceedingly
well for the one variety, and flint glass for the other. Crown
glass consists of silica, lime, oxide of iron, sometimes a little
alkali, and small quantities of other matters : these substances
are not very different in their refractive powers, and when fused
do not produce very strong streaks, even though a little differ-
ence in the composition of different parts of the glass may exist.
The glass also is not a very powerful fluxing agent upon the
crucible in which it is melted ; so that although it is in contact
with it in a fluid and heated state for many hours, it does not
dissolve much from it ; and what it does dissolve having a re-
fractive power little different from that of the glass itself, propor-
tionately less harm is occasioned. Again : the specific gravity
of the different materials used is not very different ; so that the
mixing agencies which affect the contents of the pot, such as
the ascent of bubbles, the ascending and descending currents
from difference of temperature, are more energetically ex-
erted, and the whole mass approaches nearer to uniformity in
a given time, or acquires it sooner than would happen were
greater differences to exist.

6. With plate glass the same circumstances hold nearly in an
equal degree. This substance is composed of silica and alkali
essentially, other elements being only in small quantities. Its
action upon the crucible is greater than crown glass, but then
it has a second application of heat in such circumstances as are
calculated to give a very uniform temperature to the contents
of a whole pot, and it is delivered into its final form in the
manner least likely to cause mixture of the different parts.

7. With flint glass many circumstances are altogether differ-



1829.] On the Manufacture of Optical Glass. 239

ent. Oxide of lead enters into its composition to the amount
of one third of its weight, or more, and by its presence gives
that proportion of refractive and dispersive power, which makes
the glass valuable in conjunction with crown or plate : this it
does in consequence of its own powerful action on light ; and
it makes the glass heavy also, because of its own great specific
gravity. A third property belonging to it, namely, its high
fluxing or dissolvent powers, it also confers upon the glass.
Now these three properties are unfortunately very conducive
to the formation of striae. If the least difference in composi-
tion exists between one part and another it becomes evident,
because of the great difference between the qualities of the
oxide of lead and the other ingredients ; and a variation in
proportions which in crown or plate glass would produce no
sensible effect on the naked eye, would, in flint glass, form
strong strise. Hence it is required that the mixture be in this
case far more perfect than in the other glasses ; and yet it
unfortunately happens that every thing tends to make it much
less so. The oxide of lead is so heavy a material, and at the
same time so fusible, that it melts and sinks to the bottom,
leaving the lighter materials to accumulate at the top : and so
imperfect are the means of mixture, under ordinary circum-
stances, that glass of very different specific gravity is procured
from the bottom and top of the same crucible. The following
are some cases of this kind, from pots containing glass not more
than six inches in depth, made from the usual materials, and
retained at a full heat for twenty-four hours :

Top 3-38 3*30 3-28 3'2l 3-15 3-73 3'85 3'81 3-31 3-30

Bottom. ..4-04 3'77 3-85 3'52 3'80 4*63 4-74 4-75 3'99 3'74

These differences are great, and selected for illustration;
but from appearances there is little reason to doubt that the
same state of things, though not to such an extent, occurs in
every pot of flint glass made in the ordinary way.

8. Another curious illustration of the predominance of oxide
of lead at the bottom is shown in many of our specimens, which
have been broken through vertically : they have been affected
by sulphuretted vapours and tarnished ; but the tarnish has
occurred only at the bottom, where the lead is abundant, and
is there very strong, whilst there is no appearance of it towards
the top.



On the Manufacture of Optical Glass. [1829.

9. Whilst the crucible is in the condition described, it is
clear that all those circumstances, as currents, bubbles, &c.,
which tend to mix the glass, form abundant striae and veins of
enormous strength, and do harm unless they are continued in
activity until the mixture is nearly complete ; a state rarely, if
ever, acquired in the ordinary flint glass pot. But even if this
could be the case, there is a constant cause of deterioration
arising from the highly fluxing and dissolving quality given to
the glass by the oxide of lead. In this respect, flint glass far
surpasses crown or plate glass, and it is also during one stage
of its preparation more fluid : it consequently is continually
exerting a solvent power upon the crucible to a considerable
extent, occasioning that very irregularity in composition which
produces striae, whilst the comparative levity of the matter dis-
solved at the sides and bottom, and the ascending currents at
the hottest parts of the crucible, are constantly mixing this
deteriorating portion with the general mass.

10. The difficulties which are thus introduced into the manu-
facture of flint glass fit for optical uses appeared to the com-
mittee, who, however, were none of them practical glass-makers,
to increase, as the scale upon which the inquiries were carried
on diminished : and the enormous expense of large experiments,
the time required for each, the number necessary to give
that experience which should render any one who undertook
the charge of this part of the inquiry an ordinary practical work-
man, and the uselessness of the resulting glass for any other
purpose than the one directly contemplated, compelled the
sub-committee to consider seriously on the possibility of making
other glasses than those ordinarily in use, which, at the same
time that they had the high dispersive power enabling them to
replace flint glass, might have also such fusibility as would
allow of their being perfectly stirred and mixed, and might be
retained, without alteration, in such vessels as could be procured
of any desired size.

11. The borate of lead, and the borate of lead with silica,
were the substances which, after some trials, were found to
offer such reasonable hopes of success as to justify perseverance
in a series of experiments ; and the metal platina was looked
to as the material out of which to form the vessels intended to
be used. It was soon ascertained that the borate of lead could



1829.] On the Manufacture of Optical Glass. 241

be readily formed from dry materials, and that silica might be
added with great advantage to the resulting glass ; a range of
proportions between the three ingredients being permissible,
which gave much command over the properties of hardness,
colour, weight, refractive and dispersive power, &c., and yet
remained within the required range of fusibility. Platinum also
was ultimately found to answer perfectly the purpose of retain-
ing the glass ; for though at first it was continually liable to
failure, yet it was ultimately ascertained that neither the glass
nor any of the substances entering into its composition, separate
or mixed, had the slightest action upon it. Finally, it was found
that several kinds of glass formed of these materials, were in
their physical properties fitted to replace flint glass in the con-
struction of telescopes, in some cases apparently even with
advantage ; since which time the experiments have been unre-
mittingly pursued.

12. The great proportion of oxide of lead in these glasses



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