Jökull - 01.01.2001, Qupperneq 107

Silfurbergið frá Helgustöðum
áratugum seinna en raun varð á. Menn hefðu einnig
vart verið búnir um aldamótin að átta sig á misræminu
milli rafsegulfræða Maxwells og aflfræði Newtons.
Má jafnvel spyrja sjálfan sig – meira í gamni en al-
vöru: hvenær hefðu Albert Einstein eða einhverjir aðr-
ir farið að leiða hugann að lausn á því misræmi? Af-
stæðiskenningin var sú lausn sem Einstein setti fram
1905, nýting kjarnorkunnar er síðan bein afleiðing af-
stæðiskenningarinnar, og þannig mætti telja áfram.
Heimildakönnun sú sem hér liggur að baki, er enn
harla ófullkomin og eru ábendingar um frekari gögn
tengd silfurberginu vel þegnar. Þær niðurstöður, sem
hér eru gefnar til kynna og koma nánar fram í skýrslu
minni (Leó Kristjánsson, 2001), ættu þó að vera okk-
ur Íslendingum hvatning til að veita sögu silfurbergs-
námunnar við Helgustaði meiri athygli héðan af en
verið hefur síðan 1925.
SUMMARY
Iceland spar: the unique story of the crystals from
Helgustaðir
The first record of the occurrence of certain tran-
sparent crystals at Helgustaðir farm, Reyðarfjörð-
ur, East Iceland, dates from 1668. In the follow-
ing year R. Bartholin of Copenhagen published an
essay describing their properties, among which was a
strange double refraction. The Icelandic crystals were
studied further by C. Huyghens of the Netherlands and
I. Newton around 1700. The former suggested that a
point wave source in these crystals gave rise to two
wave surfaces, one being ellipsoidal.
Not much happened in relevant fields in optics in
the 18th century, but the Iceland crystals played a part
in certain developments in crystallography in 1780–
1820, notably R.-J. Hauy’s law of rational proporti-
ons. The crystals turned out to consist of calcium
carbonate; the variety (fundamental cleavage rhom-
bohedron) which is typical for Helgustaðir but is
relatively rare elsewhere, is generally known as Ice-
land spar.
Huyghens’ suggestion was tested by
W. H. Wollaston over a century later. Wollaston’s pu-
blications in turn prompted an 1808 prize competition
by the French Academy of Sciences on the subject of
double refraction. The competition led to major disco-
veries concerning the transverse nature of light (and
heat radiation), its varied interactions with matter, lig-
ht and sound propagation in crystals, the symmetry of
crystals, and many other subjects during the next two
decades. Among scientists involved were E. L. Malus,
A. Fresnel, F. Arago and J.-B. Biot in France, and D.
Brewster in Scotland. Further theoretical and experi-
mental developments, where G. G. Stokes and F. E.
Neumann deserve special mention, occurred up to
1845. Nicol prisms which separate the two orthogonal
components of light vibrations, were invented in 1829
and soon incorporated into polarimeters and other
devices. In the meantime, Iceland spar contributed
significantly to understanding of crystal physics on
several fronts, particularly regarding the concept of
anisotropy.
In the late 1840’s Nicol prisms were a key ele-
ment in two important discoveries. L. Pasteur found
a connection between certain symmetry aspects of
crystals and their optical properties, which later
revolutionized organic chemistry. M. Faraday obser-
ved that magnetic fields affected light propagation
in matter; this observation along with J. C. Maxwell’s
own research with Nicol prisms may have hastened
the development of Maxwell’s electromagnetic theory
of light (early 1860’s).
Late 19th century developments in optics and in
optical techniques employing Nicol prisms were num-
erous, including petrographic microscopes; scattering
of light; photoelasticity; optical properties of metals
and thin films; electro–optic effects; and new mag-
netooptic effects including the Zeeman effect which
promoted understanding of light emission and atom-
ic structure. Iceland spar was the first crystal whose
elastic and inelastic deformation was studied in any
detail.
In the early 20th century Iceland spar served
as a length standard for X–ray diffraction. Ni-
col prisms were used extensively in various kinds
of instruments: polarimeters in academic, indu-
strial and medical laboratories; many crystallo-
graphic/mineralogical/petrographic devices including
reflected–light microscopes for the study of ores and
metals; photometers for measuring the intensity of lig-
JÖKULL No. 50 107