Courtesy of the Library of the Deutsches Museum, München
Giuseppe Molesini
Telescope Lens-Making
th
in the 17 Century:
26 | OPN Optics & Photonics News
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Courtesy of the NOAA Photo Library
In the early days of instrumental optics, the know-how of lens-making
was shrouded in secrecy. A remarkable exception can be found in a
letter from 1643, in which the Italian lens-maker Vangelista Torricelli
gives his correspondent directions on optical shop practices.
t the dawn of the 17th century, optical technology was almost exclusively devoted
to mirrors and spectacle lens making. Spectacles had been around since the late
13th century for correcting farsightedness, but the introduction of lenses for correcting myopia was relatively recent in the 1600s. The availability of both positive and negative
lenses laid the groundwork for the advent of the telescope.
At that time, optical shop practices to produce spectacle lenses were well established and
known to artisans in various European cities. However, when it came to telescope lensmaking, and particularly the objective, the inadequateness of the fabrication process became
evident. Although Galileo succeeded in obtaining high-quality telescope lenses, it was the
result of a careful selection among many pieces that had been fabricated by the best artisans
of the time. It was only around the middle of the 17th century that telescope lens-making
evolved into a reliable process in which quality lenses could be obtained consistently as a
result of specific fabrication procedures.
(Facing page) Jan van der Straet’s “Spectacles” (1584). On the left of the image, a vendor sells
spectacles in a European marketplace. (Above) Vangelista Torricelli, as represented in 1715 on the
frontispiece to a collection of his academic lectures.
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1047-6938/10/04/0026/6-$15.00 ©OSA
Torricelli started his
lens-making work
in 1642, and it did
not take him long to
achieve full mastery
of his craft. To this
day, optical testing
is still carried out
on some of his
remaining lenses.
The passage from spectacles to telescope lens making is not known in detail, as documents and descriptions are missing in the historical record. Most steps probably occurred
in the shops of unknown artisans who kept their achievements secret in order to maintain a
competitive edge. As a first and remarkable exception, a letter from Vangelista Torricelli, a
lens-maker from Florence, details the state of the art in telescope making to his correspondent Raffaello Magiotti in Rome. The letter is dated December 4, 1643.
In the standard
craft of 17th century
spectacle making,
lenses were obtained
from glass plates or
pre-forms; a circular
shape was created
by edge chipping
with cutters
and pincers.
17th century lens-making
28 | OPN Optics & Photonics News
Who was Vangelista Torricelli?
Vangelista (or Evangelista) Torricelli was born in Rome in 1608. He was a disciple of Benedetto Castelli, who was in turn a follower of Galileo. Among his major achievements are the
discovery that air has a specific weight, and that a vacuum is formed inside a tube of appropriate length filled with mercury—a finding that pioneered the invention of the barometer.
In fact, the Torr (the pressure unit that is equal to 1/760 of a standard atmosphere) is based
on Torricelli’s name.
Torricelli was invited to Florence in 1641; he assisted Galileo during the last three
months of Galileo’s life. He was then nominated to the position of Mathematician of the
Grand-Duke of Tuscany, and he continued his studies until his premature death in 1647
at the age of 39.
Torricelli started his lens-making work in 1642, and it did not take him long to achieve
full mastery of his craft. To this day, optical testing is still carried out on some of his
remaining lenses. Making large high-quality, long-focal-length lenses was a significant task
at that time. The court of the Grand-Duke was interested in maintaining its supremacy—
which was rooted in the telescope of Galileo and the fame of his discoveries. Torricelli’s
approach to telescope lens-making built upon spectacle-production technology; having a
solid scientific grounding, he could single out the critical points of the process and devise
appropriate remedies.
In the standard craft of 17th-century spectacle-making, lenses were obtained from glass
plates or pre-forms; a circular shape was created by edge chipping with cutters and pincers.
Roughing was made with wet sand on appropriate tool plates, so that most of the excess
material was removed and the surfaces acquired a figure close to the final one. More precise
grinding to the desired figure and reduction of the roughness was made with spoltiglia (a
slurry of emery powder in water that is made by successively eliminating the sediment) on
a metal tool plate.
To this purpose, the lenses were attached to a wooden handle by means of hot pitch. The
artisan had to press and drive the handle against the tool plate until the pits and scratches
produced at roughing were consumed. The final polishing was done by scrubbing against a
cloth smeared with tripolo (a fine powder of silicon dioxide). The operation was carried out
by hand, or with the help of a turning machine that rotated a disc on which the cloth was
secured. At the end, the lens was detached by imparting a sharp stroke to the handle.
The process described by Torricelli in his letter differs from the one I just described
mainly in the grinding stage. As for the tool plate, it consisted of a disc of glass, of the same
size or a bit larger than the lens to be ground. The tool plate was preliminarily deepened
by means of a smaller disc of glass with sharp emery, abrading more about the center than
the edge. The shape of the tool plate did not need to be particularly regular because, due to
wear, “Nature itself makes it most perfect, where the art could never attain.”
As for the lens, it was attached to a handle. However, the handle was significantly lower
and wider than it is in the art of spectacle making, being driven by the palm instead of the
fist. Torricelli used a muller of lead, so that its weight relieved the artisan from pressing.
The grinding action with spoltiglia was continued until the roughing marks disappeared
and beyond, with the artisan moving the muller around in a random manner and describing “irregular and extravagant movements, i.e., coils, scrolls, circles, and mostly diameters,
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many and in all directions.” The operation was only terminated when the remaining
spoltiglia became greasy—“like butter.” The lens surface was then lightly polished with
tripolo on stretched cloth against a hard background, just to remove the remaining haziness;
Torricelli realized that the figure does “matter very much, and the polishing very little.”
The size of the tool plate at the grinding stage was approximately equal to that of the
lens, and this was a significant detail: In this condition, the random movements imparted
by the artisan were such that the two surfaces being abraded—the lens and the tool plate—
formed a pair of spherical surfaces, one concave and the other convex. The latter process
was clearly understood by Torricelli, being based on the geometrical properties of the sphere
as the locus of the points at the same distance from the center: Wear in fact takes away
the bumps and valleys and equalizes the surfaces; he could then say that “neither an Angel
could impart to the glass a figure more perfectly spherical.”
In the letter, Toricelli bestows some secret particulars to Magiotti, the major one being
his use of a small tool plate to generate spherical surfaces and his advice “not to use fire.”
The latter refers to the standard practice of attaching the lens to the muller by means of hot
pitch, thereby causing lens bending by thermal expansion at the blocking stage, followed by
a release of the strain when the lens is detached. Torricelli recommends instead the use of
wax, which is a cold glue.
History reports of a final secret, regarding a subject that had only been confided to the
Grand-Duke. A document of the time reported by Paolo Galluzzi states that Torricelli’s last
secret consisted of polishing the lens on a thin plate of lead or tin laid on the same tool plate
that served for grinding. Such an approach was similar to the innovation that was attributed
to Newton in 1721—using a pitch layer for polishing.
Without further ado, I present Torricelli’s letter, which reveals so much of the optical
shop practices of the time. It is one of the most fascinating documents even written about
optical technology.
Note: The authors comments are in brackets.
lease then know that the tool plate is very easy to make: and Nature itself
makes it most perfect, where the art could never attain.
One takes a piece of flat glass, or raw, round, and just large as much as the glass
to be worked, or a little bit more. One attaches to it something heavy, so that the
hand does not drag it around during the work; I use a disc of lead, or a brick or other
thing. After this, I start to deepen it with a small glass also flat and sharp emery.
While deepening it I don’t keep anything else than that the glass, by which I deepen
it, passes more often about the center than the edges of the future tool plate. On
the whole, before one hour has passed (even if the glass were raw) I have deepened a
tool plate for a telescope of three arms and half [approximately 2 m], worked out on
both sides [equiconvex objective]; intending though that the tool plate should have
a diameter not larger than 1⅔ florentine piastra [approximately 7 cm; piastra was a
silver coin about 42 millimeters in diameter].
Please don’t have scruples about the tool plate, because it suffices that it is deepened
grossly, and then working the glass it is made perfect by Nature itself. Once this is
done, the small glass that deepened the tool plate is put aside, and one takes the glass
that has to be worked out, well rounded, and also rough-ground in a spare tool plate
of copper or other material, provided that it is not completely flat, nor deep to the
point of being out of proportion with respect the tool plate already prepared. This
[the glass] is then started to be worked with fine spoltiglia, until Your Lordship judges
that it has become fitting to the tool plate; such thing [the fitting] is recognized
Torricelli’s last
secret consisted of
polishing the lens on
a thin plate of lead
or tin laid on the
same tool plate that
served for grinding.
Such an approach
was similar to the
innovation that
was attributed to
Newton in 1721—
using a pitch layer
for polishing.
“Please don’t have
scruples about the
tool plate, because
it suffices that it is
deepened grossly,
and then working
the glass it is made
perfect by Nature
itself.”
[continued]
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“When then the
glass is ground
everywhere, one
stops adding
spoltiglia, but
continues working
with the residual
that has remained
between one glass
and the other, and
also on the rim.”
“As to the
smallness of the
glass tool plate
said above, i.e.,
that it has to be
equal to the glass
to be worked,
Your Lordship has
to consider it a
great secret.”
even at sight, because the glass that was rough-ground with the emery had the grain
coarse, but after, where it will have got a taste of the spoltiglia, it will have it [the
grain] more minute.
When then the glass is ground everywhere, one stops adding spoltiglia, but
continues working with the residual that has remained between one glass and the
other, and also on the rim. Such an operation is continued until that matter is
consumed, and turned white, impalpable, and greasy like butter, wetting the tool
plate (when it would become dry) with half a droplet of water, or with the breath of
the mouth put there near. If such operations are done well, the glass will result free of
scratches, and of marks, and will have a finish such that, placing it to the axis of the
eye at about half a right angle, it will mirror the luminous things.
As regards polishing, never one polishes [the glass] on the same tool plate that has
worked it, because it gets polished first at the edge, and then very late in the middle,
and they [the glasses] do not turn out well. One has then to use a milder tool plate. I
use a disc of slate of approximate width of eight fingers, and almost flat. I just give it
four [i.e., a few] strokes with pumice, until the eye starts to perceive that it is no more
flat. I put this on a table with a disc of cloth underneath it, in order that it does not get
broken, and then I fix above it, with small nails of the kind used for window covering,
a piece of fine cloth free of knots, moths etc. and stretched on all sides as much as
possible. This invention is better than fastening the cloth around the tool plate, because
one stretches it better; and then because, being the cloth secured to the underlying table,
the tool plate remains at rest under the turns of the hand. The tripolo is then applied in
the form of ointment so scarce that it does not mass about the edges of the glass and the
tool plate, adding now a few droplets of water, then a little tripolo, as the cloth will need.
One only has to have a little patience with polishing, because any slightest ugliness and
unevenness in the glass surface goes away.
As to the smallness of the glass tool plate said above, i.e., that it has to be equal to
the glass to be worked, Your Lordship has to consider it a great secret. I think you will
understand very well that if the tool plate is not spherical, neither the glass can be of
good sphericity. And who assures me that the tool plate remains spherical, when the
plate is one palm in diameter, and the glass one piastra [i.e., much smaller]? But when
they are equal, and the hand of the artisan makes irregular and extravagant movements,
i.e., coils, scrolls, circles, and mostly diameters, many and in all directions, well then
neither an Angel could impart to the glass a figure more perfectly spherical.
The secret then that I am concerned with, and which is known by nobody else
than God and myself, is this. Never attach the glasses to be worked with pitch nor
other by means of fire, because in the cooling those materials shrink more in one side
than the other, and bend the glass, which, as far as it stays attached to the muller,
has a best figure, but when we detach it to place in the telescope, it gets flattened as
before, and the figure is spoiled. This secret that I am now telling Your Lordship has
[continued]
The author is grateful to P. Galluzzi for critically reading the text and for useful suggestions, and to
I. Truci for her help with bibliographic research.
30 | OPN Optics & Photonics News
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been noticed by myself with so much of evidence, that I have touched it by hand, and
could tell Your Lordship also how, but I skip it for brevity. Now I attach the glass in
this manner: I take a muller of lead of this proportion;
Courtesy of the Riccardiana
Library, Florence
to the flattened side A I apply a disc of inside or other fine yielding cloth, so that the
glass touches against a soft layer; then above the said cloth I dress the muller with a
very stretched glove skin and I fasten it most tightly with the string CD: then I daub
the side of the said skin A with red wax, warm and spread very thin. In this manner
the glass (unless it is wet) will always attach provided it is well cold; and in case it is
needed, one gives a rub to the said skin with a ball of the same red wax, which will
attach very strongly.
As a consequence the glass is not going to be strained, but when it is detached from
the muller it maintains the same figure it receives from the tool plate. In addition to
this Your Lordship will have the convenience of starting to test the glass if it works
well or badly as soon as one starts to polish, and will have the possibility of detaching
and attaching it hundred times without any damage, and rather with benefit. Since
when one uses the pitch, the rule is not to detach it until it is finished. As regards the
invention of the muller of lead it is not mine, but it is very good; because when doing
the fine grinding it is not necessary to push down the hand almost at all, but the lead
itself does it almost alone; also in the polishing it helps a lot: and so that it better makes
its service we have the mullers that are almost two inches more in diameter than the
glass itself, so that they press down somewhat more. And please notice that making
the muller very tall is bad because it plays lever and makes the glass to lose its balance.
When Your Lordship will try these inventions (which are only two: small tool plate and
not to use fire) I assure you will make good glasses, even if the bulk material were bad
[sic]; and none will ever turn out bad at all, but always more than middling. As to the
very good ones, then, they are lucks; and it is necessary to bring together many things,
the figure, the material, the polishing. The observation has taught me that in the
glasses the figure matters very much, and the polishing very little. The reason is this.
I have tested many glasses of mine that were just starting to get transparent, and I have
seen that in spite of the very coarse grain they had, they anyway did well because the
figure was good. Others then, polished like diamonds, because of a bit of unimaginable
failure that is in the figure, they do nothing. I ask you to keep secret what I write you,
in particular that of attaching, because it is a thing no one suspects about, and there is
nothing that spoils more the glasses, when though they are not very big.
Member
[ References & Resources ]
>> S. A. Bedini. “Lens making for
>>
>>
>>
>>
>>
scientific instrumentation in the
seventeenth century,” Appl.
Opt. 5, 687-94 (1966).
P. Galluzzi. “Evangelista
Torricelli—Concezione della
matematica e segreto degli
occhiali,” Nuncius 1, 71-95
(1976).
V. Greco et al. “Telescopes of
Galileo,” Appl. Opt. 32, 621926 (1993).
M. Miniati et al. “Seventeenthcentury telescope optics of
Torricelli, Divini, and Campani,”
Appl. Opt. 41, 644-7 (2002).
G. Molesini. “The Telescopes
of Seventeenth-Century Italy,”
Opt. Photon. News 14(6), 34-9
(2003).
V. Ilardi. Renaissance Vision
from Spectacles to Telescopes,
American Philosophical Society, Philadelphia, 2007.
Giuseppe Molesini ([email protected]) is with the Istituto Nazionale di Ottica Applicata,
in Florence, Italy.
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