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Serendipity and a Spider
William Gascoigne
(c.1612-44) and the Invention of the Telescope Micrometer
As the French astronomer Adrien Auzout penned a letter to the Royal Society of London on 28th
December 1666 he could not have imagined the consternation that it would cause.
Nor could he have guessed that his action would almost certainly save from
oblivion the work of a young astronomer from northern England, who had suffered
a violent death almost a quarter of a century earlier.
Full of enthusiasm, Auzout wanted to
tell English scientific friends of his invention of the telescope micrometer:
an entirely novel device that, together with Jean Picard, he had used to
measure the angular diameter of the Sun, Moon and planets. “We can take
diameters to seconds”, he wrote, “…. and we can be almost certain that we
cannot be mistaken by more than 3 or 4 seconds …. I can well assure you that
the diameter of the sun was hardly smaller at its apogee than 31' 37" or
40"”
When Richard Towneley, a wealthy
Lancashire patron of science, saw Auzout’s letter in
the Philosophical Transactions of the Royal Society, he was dismayed. He
contacted the Society: “I am told I shall be looked upon as a great wronger of our Nation, should I not let the World know,
that I have out of some scattered Papers and Letters that formerly came to my
hands, of a Gentleman of these parts, one Mr Gascoigne, found out, that before
the late unfortunate wars, he had not only devised an Instrument of as great a
power as Monsieur Auzout’s, but had also for some
years made use of it …. The very Instrument he first made, I have now by me”.
Towneley’s letter
appeared in the next issue of the Transactions. Auzout,
though keen to have more details, took the news with good grace.
Of the life of William Gascoigne (1612-44), the Yorkshire
astronomer whose micrometer and observations had fallen into the hands of Towneley, little was known. He was born into a wealthy
land-owning family and lived at New Hall in the tiny Yorkshire village of
Middleton, near Leeds. He made his own lenses and telescopes, and invented a
telescopic sight and a telescope micrometer. Before his untimely demise, he had
a Treatise on Optics ready for the press.
Though by his own account spending on astronomy only the same time
as friends and neighbours spent on “hawks and hounds”, by the age of 28
Gascoigne had made some remarkable innovations. He had devised experiments to
show how an inverted image was cast onto the retina of the eye. He had verified
the sine law of refraction – recently published by Descartes – and had used the
same law to calculate the angular size of the Sun, from the size of its image
projected by a Galilean telescope.
One day, returning to the open case of a Keplerian
telescope with which he had been experimenting, Gascoigne found that a spider
had spun its web inside the case – a few centimetres from the convex eyepiece.
With characteristic curiosity, rather than sweep the web aside, he bent to look
through the telescope. To his astonishment, he saw that, not only was the web
in focus, so too were distant objects.
“This is that admirable secret, which, as all other things,
appeared when it pleased the All Disposer, at whose direction a spider’s line
drawn in an opened case could first give me by its perfect apparition, when I
was with two convexes trying experiments about the sun, the unexpected
knowledge.”
In the Keplerian telescope the objective lens was convex and
unlike in the Galilean model so too was the eyepiece lens. With this
arrangement any object placed at the focal point of the objective lens, could
be magnified and brought into focus alongside the image of the distant object
being viewed. This ‘incredible rarity’, as Gascoigne called it, immediately
suggested a number of possibilities. Firstly, it was feasible to introduce a
simple marker, to act as a telescopic sight.
“if
I .... placed a thread where that glass [the eyepiece]
would best discern it, and then joining both glasses, and fitting their
distance for any object, I should see this at any part that I did direct it to ...”
Secondly, one could
introduce a measuring device into the common focal point and thereby measure
the size of the observed image. At first he used simply a scale. Then he
introduced a pair of brass pointers. These were mounted on a screw and could be
moved towards and away from each other by turning the screw (Fig.1). The change
in separation was proportionate to the number of turns, as also was the angular
separation of the two distinct points enclosed by the pointers. With this
ingenious device he measured the diameters of the sun, moon and planets.
Fig.1
Gascoigne's telescope
micrometer
Gascoigne was in regular contact with the young Lancashire
astronomers, William Crabtree and Jeremiah Horrocks
and, for a remarkable, though lamentably brief period, these three blazed a
trail for precision astronomy in Europe.
Whilst William Crabtree, however, was using something similar to a
cross-staff, fitted with needles, to observe the sun through thin cloud and
measure its angular diameter, Gascoigne, by contrast, was using the “wheels and
screws” of the micrometer to make similar measurements with unprecedented
accuracy. The whole purpose was to test Kepler’s idea
that shape of the Earth’s orbit was an ellipse, rather than the eccentric
circle argued by Copernicus.
From ancient times it had been assumed by astronomers that the
distance between the Earth and the Sun varied over the course of the year
(Fig.2). Hipparchus had deduced this
from the observed difference in the length of the seasons and an assumption
that the Sun moved around the Earth at a uniform speed. Copernicus, though he placed the Earth in circular motion around
the Sun, was obliged to offset the centre of the circle from the Sun to get the
right length for the seasons. Kepler introduced
the notion of an elliptical orbit of the Earth, with the Sun at one focus of
the ellipse. All these theories implied a variation in the apparent angular
diameter of the Sun throughout the year. The variation of this apparent
diameter and hence the ‘eccentricity’ of the orbit, however, was far too slight
to be measured reliably - until, that is, the invention of the telescope
micrometer. Herein lay the importance of the device invented by Gascoigne and Auzout. The precise value of the eccentricity could be
calculated from the semidiameters at the furthest and
nearest approach of the Earth to the Sun. What is more, they would differ,
according to whether Copernicus or Kepler was
correct.
Fig.2 Three alternative models for the orbital
eccentricity in the Earth-Sun system.
a=maximum distance
of the orbiting body from the centre of the of orbit
e= the eccentricity.
The offset of the Earth (geocentric theory) or the Sun (heliocentric theory)
from the centre of the orbit, expressed as a fraction of a
SS= summer solstice, WS=winter solstice, AE=autumnal equinox, VE=vernal
(or, spring) equinox
Gascoigne’s values of 15' 53" and 16' 25", respectively,
implied an orbital eccentricity of 0.01651 (compared with the current value of
0.01672), and helped to confirm the elliptical orbit of Kepler.
In 1642 England was plunged into a bloody civil war, which brought
an end to Gascoigne’s astronomical pursuits. We will perhaps never know what
impelled William Gascoigne to join the King’s side in the conflict. Whatever
the case may have been, he abandoned his telescopes and – despite the
impediment of a lame leg - threw in his lot with the Cavalier army of Charles
I.
Within two years he was dead - killed by a bullet, according to
Christopher Towneley, at the battle of Marston Moor
near York on 2nd July 1644: His naked body probably thrown into a
mass burial pit along with up to 10,000 other unfortunate young men.
RESCUED
FROM OBLIVION
Even after Richard Towneley’s belated
account of Gascoigne’s inventions it appeared that his contributions
were destined to fade into oblivion. A
handful of letters between Gascoigne and Crabtree were in the hands of the Towneley family, until 1711 when they were handed to the
Reverend William Derham.
Shortly after getting them, Derham
happened to notice an obscure passage in the Preface to 1687 tables of Philippe
de La Hire (1640-1718), in which the invention of the telescopic sight was
attributed to Picard. Derham promptly wrote to the
Royal Society in London, saying “Monsieur de La Hire … robs our Mr Gascoigne of
the honour of first applying telescopic sights to mathematical instruments and
ascribes it to Monsr Picard …. Now since Mr
Gascoigne’s papers are in my hands … I am fully able to do Mr Gascoigne
justice”.
When Derham’s paper was published, it
demonstrated beyond doubt, with extracts from letters of Gascoigne and
Crabtree, that Gascoigne had not only invented the telescopic sight and
micrometer, but had also taken many measurements with them.
Meanwhile,
de la Hire was busy compounding his ‘misdemeanour’. In a new memoir concerning
the date of the invention of the Micrometer he did not even mention Gascoigne.
Instead, he said “we are indebted to Monsieur Huygens for the invention of the
micrometer”.
Across
the Channel it took nearly four decades for a response to come. By then John
Bevis – the discoverer of the Crab Nebula – had found a new piece of evidence
confirming Gascoigne’s achievements. In the library of the Earl of Macclesfield
he had found and copied a letter written by Gascoigne to the mathematician,
William Oughtred, in 1640-01. “It consists of several
sheets of paper”, he reported to the Royal Society, “all about his invention
for measuring small angles to seconds; where he not only gives the geometrical
and optical principles of his contrivance, and the construction of the
instrument, but also a series of observations actually taken therewith.”
Eventually, the true role of Gascoigne was acknowledged on both
sides of the Channel. In his authoritative History
of Modern Astronomy (1821) the French astronomer, Jean-Baptiste
Delambre devoted several pages to Gascoigne and the
priority dispute. He acknowledged the Middleton astronomer’s micrometer
measurements of the solar disc and compared them favourably with the
theoretical values given in the French national almanac, Connaissance des Temps.
Gascoigne Connaissance
des Temps
25 October (OS) 16' 11" or 10" 16' 10"
31
October 16'
10" 16'
11".4
2
December 16'
24" 16'
16".8
Horrocks and Crabtree achieved lasting fame
because of their observation of the 1639 transit of Venus. A fame which is
reinforced with each fresh transit ‘season’, including that through which we
are fortunate to have lived (2004-12). Their friend, William Gascoigne –
according to Flamsteed, “as ingenious a person as the
world has bred or known” - has been consigned, however, to a mere footnote in
the history of astronomy.
The original letters that Derham
acquired have long since been lost. Ironically, it is largely due to the
unintentional ignoring of Gascoigne’s achievements by French astronomers (Auzout, de La Hire) that Gascoigne was rescued from total
oblivion. Without the resulting priority dispute, we would
probably be unaware of the tragic story of that remarkable young man.
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Note: David Sellers is the author of The Transit of Venus: the Quest to Find the True Distance of the Sun
(Leeds, 2001), co-author of Vénus devant le Soleil (Paris, 2003, Ed. A Simaan), and the author of In Search of William Gascoigne: Seventeenth Century Astronomer
(Springer, New York, 2012)
FURTHER READING
Sellers, David, In Search of William Gascoigne: Seventeenth Century Astronomer, Astrophysics and Space Science Library, Volume 390. ISBN 978-1-4614-4096-3. Springer Science+Business Media New York (2012) - see publisher's flyer
Sellers, David, A letter from William Gascoigne to Sir Kenelm Digby, Journal for the History of Astronomy (ISSN 0021-8286), Vol. 37, Part 4, No. 129, p. 405 - 416 (2006)
Chapman, Allen, Three north country astronomers (1982), Dividing the Circle (pp. 35-45) (1995), Oxford Dictionary of National Biography (pp. 591-3) (2004)
Willmoth, Frances, William Gascoigne, in Encyclopedia of the Scientific Revolution (ed. Wilbur Applebaum) (2000)
Thoren, Victor, William Gascoigne, in Dictionary of Scientific Biography (ed. C.Gillespie) (1972)
Taylor, R.V., Biographia Leodiensis (pp.86-7) (1865)
Wheater, William, William Gascoigne - the astronomer, The Gentleman's Magazine (pp.760-762) (1863)