Sky & Telescope - July 2024 - 32

THE PICTURE ART COLLECTION / ALAMY STOCK PHOTO
Astro-History
perfectionistic, morose individual. At the time, he was busy
trying to improve the quality of flint glass for use in achromatic
lenses, which were just being developed. Unfortunately,
the material was difficult to cast without introducing streaks
and veins, which compromised the lenses' optical quality.
Even more discouraging, an exclusive patent for achromatic
telescopes had been granted to Peter Dollond of London,
which effectively gave Dollond right of first refusal on the best
optical flint glass available. Dollond's patent and the scarcity
of good optical glass made achromatic telescopes at the time
prohibitively expensive. As a result, a great deal of experimentation
went into the development of speculum-mirror
reflectors, such as those made by Michell and Herschel. Even
Boulton performed experiments, successfully demonstrating a
new method for making speculum mirrors in 1778.
In a few years, Watt migrated from working with lenses
and telescopes to steam engines, and in 1774 he moved to
Birmingham to develop his ideas with the aid of Boulton's
capital. Although he didn't invent the steam engine, Watt
introduced various improvements. One involved a " sun and
planet gear, " which converted vertical motion into rotatory
motion. It was so-called because it resembled Ptolemy's
description of planetary motion in the Almagest but also -
and perhaps this was its real inspiration - some of the gear
mechanisms Ferguson employed in his orrery. Although the
sun-and-planet gear was actually invented by the Scottish
engineer William Murdoch, Watt recognized its importance
and was first to patent the idea.
The Circle Expands
Through their mutual interest in building a canal to connect
the Trent and Mersey rivers, Darwin collaborated with Josiah
Wedgwood, who was in the process of developing a large pottery
factory at Etruria, in what is now Stoke-on-Trent, Staffordshire.
Wedgwood's factory lay directly in the path of the
proposed canal, which would provide a crucial connection to
the markets for his pottery.
Yet another force of nature joined the group when Michell
introduced Darwin and Wedgwood to Joseph Priestley, then
serving as minister to Mill Hill Chapel, in Leeds. Sharing
the other Lunar Circle members' fascination with electricity,
Priestley frequently traveled from Leeds to Birmingham to
attend meetings mostly held at Boulton's home, Soho House.
By 1775, the informal association of learned friends
adopted the name Lunar Society. Their only real goal was to
convene to engage in stimulating conversation and brainstorm
new ideas. In addition to Watt, Priestley also relocated
to Birmingham and quickly became the Society's most active
member, replacing Boulton, who was increasingly preoccupied
with developing Watt's steam engine. Among Priestley's
many achievements were the independent discovery of oxygen
in its gaseous state (which he called " dephlogisticated air " ),
extensive writings on electricity and - perhaps his greatest
boon to humankind - the invention of carbonated water,
which Johann Jacob Schweppe later developed into what was
to become a multibillion-dollar soft-drinks industry.
The Lunar Society members engaged in numerous activities,
which frequently overlapped. For example, the problem
with imperfections in flint glass caught the attention of
Wedgwood, though his interest was only secondarily astronomical.
Much more important to him was the material's use
in ceramics. For several years, beginning in 1779, he carried
out a series of experiments that eventually led to an important
discovery: By constantly stirring the molten flint glass,
the streaks and veins that affected its use in optics could be
eliminated. Unfortunately - possibly because he wanted to
avoid paying a large excise duty on the quantities of glass he
had used in his experiments - Wedgwood never published
the paper he wrote on the subject. And so, the secret of making
high-quality flint glass for achromatic lenses remained
unknown until the Swiss glassmaker Pierre Louis Guinand
independently discovered it 15 years later. Guinand's glass,
combined with the lens-designing genius of the Munichbased
optician Joseph von Fraunhofer, would revolutionize
observational astronomy in the 19th century. But for the
excise tax and Dollond's patent, the British rather than the
Germans might well have taken the lead in introducing the
age of the refractor!
THE ANGER OF THE MOB Priestley's liberal views about politics and
religion were widely seen as subversive in England after the excesses of
the French Revolution and led to his home being looted and set on fire
by an angry mob in July 1791.
32 JULY 2024 * SKY & TELESCOPE
A Society on the Wane
The Lunar Society inspired widespread interest, even among
those who weren't part of the group's core membership. In
the society's outer orbit was the artist Joseph Wright of Derby,
who specialized in dramatic scenes in which moonlight often
provided the illumination. Another was artist John Russell
of London, who borrowed a Herschel reflector and used it to
produce numerous precise sketches of the Moon, including
the one presented on the facing page. Appropriately, one of his
pastels hangs today in the study of Soho House.
Sky & Telescope - July 2024

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