Download Iceland spar

Iceland spar and its early applications in
microscopes and other optical equipment
Leó Kristjánsson
Institute of Earth Sciences
University of Iceland
Plenum lecture given at the Scandem2009 conference on
microscopy, Reykjavík June 2009 (with minor modifications)
Visitors to Iceland perhaps ask themselves...
...did this remote and sparsely populated island ever
contribute anything significant to the outside world ?
Yes, at least in one case which in fact has much to do
with microscopes, crystals, materials science, and optics
It is Iceland spar, “...der isländische Doppelspat, dieses
unvergleichliche Mineral...der Grundstein unserer physikalischen
Kenntniss der Krystalle...” (G. Tschermak, 1881)
Our story begins in 1668, when the King sent a stone-mason to East
Iceland to recover crystals found at a site close to a local trading post
O
Utstekkur farm, Reydarfjördur. Photo: Mats Wibe Lund
The crystals were studied by the scientist Rasmus Bartholin,
who published a booklet on his observations in 1669
The dot A under this transparent
crystal is seen as two dots
For one image C, the light ray
obeys Snell’s law of refraction
The “extraordinary” image B
rotates with the crystal
Optics, crystals, etc., 1670-1710
• This phenomenon of double refraction was noted in some
other minerals, but is difficult to observe in most of them
• A chapter in Chr. Huygens’ book Traité de la Lumière,
1690, dealt with this phenomenon. He showed that the
properties of light changed on traversing Iceland spar
• Huygens assumed light to be a wave. A point source of
light within a crystal like Iceland spar would give rise to
two waves, one circular and the other ellipsoidal
• Isaac Newton, in contrast, expected light to be a stream
of particles in his “Opticks” 1704-06.
Most scientists accepted
Newton’s view to 1800.
Only a few papers on
double refraction appeared
in 1710-80, this one in 1761
The subject was revived
by the “father of
crystallography”, the abbot
R.J. Haüy, in 1780-1805
after experiments with
Iceland spar.
Light was demonstrated to
be a wave in Th. Young´s
slit experiments, 1802
Iceland spar and optics, early 19th century
• E.L. Malus discovered in 1808 that reflection from smooth nonmetallic surfaces changed a light beam in much the same way as its
passage through Iceland spar. He called it “polarization” of light
• D. Brewster found experimentally for many materials in 1811-15,
that this polarization by reflection was complete at an angle of
incidence i = arc tan(n)
• F. Arago saw in 1811 brilliant colors in
sheets of mica viewed through Iceland spar.
• This “chromatic polarization” which was
shown by Th. Young in 1814 to be due to
interference of polarized waves, became
the foundation of petrographic microscopy
D. Brewster
Brewster realized that some crystals have one axis of
symmetry for light propagation, others have two such axes
The interference figures appearing in crystal plates in convergent polarized
light have been valuable in the identification of crystals
Uniaxial crystal
Biaxial crystal
A. Fresnel’s experiments on
diffraction further confirmed the
wave hypothesis. In the 1820s he
then developed equations for the
reflection and refraction of light
waves at boundaries, as well as for
the propagation of light in
crystalline materials
Fresnel realized that in order to
explain double refraction, light had
to be a transverse wave
His theories were tested several
times, with the aid of Iceland spar
A. Fresnel
What was so special about Iceland spar, which is only
a variety of the common mineral calcite (CaCO3)?
• Calcite has unusually high birefringence
• The Iceland spar crystals were often:
– Large and homogeneous
– Very pure (ca. 99.95%)
– Very transparent, and free from cracks, twinning etc.
– Easily cleaved, leaving very clean smooth surfaces
How did the crystals find their way to scientists?
- with local merchants and traders
- on fishing vessels from Europe
- with occasional travellers
Mining of the crystals for export began in 1850, and continued
intermittently to 1925. The major effort was in 1863-72
The crystals were deposited by hydrothermal
alteration of basalt lavas within a large (eroded)
volcanic center of about 11 M.y. age
The mine, now
under protection,
is ca. 30 m wide
and 10 m deep
Small crystals can
still be seen there
Around 1830, scientists realized that the polarization
state of light provided a very sensitive means for
probing the nature of matter
Using an Iceland spar rhomb for such
observations was sometimes inconvenient,
due to image overlap
So in 1828, W. Nicol invented an Iceland spar
prism where one image was discarded.
Thousands of Nicol prisms have been made.
Nicol prisms are
also valuable in
finding the direction
of vibration in
polarized light.
In the 1830s, the first microscopes equipped with calcite rhombs or
Nicol prisms were described, e.g. by G.B. Amici
Apparatus with glass- mirror polarizers was also employed for studying crystal plates
Spar
Amici 1832/1844
Nörrenberg 1842
Microscopes for the study of crystals
developed gradually from the 1830s
 Nicol prism
The type on the right was in use around
1860, for instance by A. Des Cloizeaux
Powell 1841
In 1863 F. Zirkel combined H.C. Sorby’s technique of making thin
sections of rocks, with a microscope containing two Nicol prisms
These lithographs
are from an 1876
paper by Zirkel
Biologists also began to appreciate by 1860,
that polarized light brought out previously
invisible structures in tissues
One example is E. Brücke’s microscope
picture of striations in muscle fibers, 1858
F. Zirkel
A main reason for the use
of polarized light in
microscopy is that
differences in the
birefringence of minerals
appear as different colors
This was the discovery
made by Arago in 1811
Gabbro. Top: ordinary light, bottom: crossed nicols
Polarizing microscopes, made commercially for the study of thin
sections in parallel or convergent light, appeared in the 1870’s
Advantages of using polarized light in microscopes for
mineralogical/petrographical studies, ca. 1880 -1900
• It offered a fast way of identifying and analysing minerals
• Many minerals previously thought to be rare, were in fact common
• Associations between minerals were useful in prospecting
• Alteration, zoning, twinning, strain effects etc. became evident
• With heated stages, phase transitions could be studied up to 1500°C
• The technique aided in synthesis of minerals and rocks
Projector, 1875
Specialized instruments for
the study of individual
mineral grains were designed,
as well as teaching equipment
Axial-angle apparatus, ca. 1880
In the mid-1880s, stocks of Iceland spar from 1863-72
were exhausted, and extraction of optical-quality
crystals at the East Iceland site also became difficult
This caused a “spar famine” among many users for decades,
and led to attempts at finding other mines or substitutes
From around 1900, minor quantities of (mostly small)
usable calcite crystals were obtained from various other
locations e.g. in Crimea (Russia), U.S.A. and Germany
Sites in South Africa became the chief supplier after 1920
In spite of the “spar famine”, the development of
petrographic microscopes continued...
Swift/Dick
1889
A. Nachet
1900-10
R. Fuess
ca. 1900
E. Leitz
ca. 1905
Bausch & Lomb
ca. 1910
C. Reichert
1909
.....and continued
Photo: D. Kile
Zeiss/Winkel 1918, universal stage
Leitz 1919, with reflected-light and other options
What about the results ?
“...und nur der mikroskopischen Untersuchung dankt [die Gesteinskunde]
die ungeheuren Fortschritte, welche sie im letzten Drittel des 19.
Jahrhunderts zu verzeichnen hatte.” (E. Weinschenk, 1901)
Other uses of polarizing microscopes to the 1920s included
- Industrial materials: Portland cement, slags, abrasives, paint fillers,
refractory compounds for furnaces, glass, ceramics,....
- Micro-chemical analysis
- Liquid crystals (for which polarized light was essential), from 1888
- Biological materials: bones, fibres, muscles, shells, gallstones,...
- Ore minerals, using reflection of polarized light (after 1922)
Another important application of Nicol prisms from Iceland
spar was in polarimeters, which measure optical activity
It occurs in some crystals and in various organic liquids and solutions.
The direction of polarization
of light rotates on passage
through these substances.
Polarimeters were available
commercially before 1850.
A typical polarimeter with two Nicol prisms, 1874. An adjustable
quartz wedge compensates for rotation caused by a fluid such as a
sugar solution in a long glass tube (not shown)
Polarimeters were mostly employed in organic chemistry
- A prime discovery was made by
L. Pasteur in 1848: optical activity
is due to a mirror-image asymmetry
of molecules. This led later to the
realization that the valency bonds
of carbon are three-dimensional
- Structure of sugars and other carbohydrates, e.g. by E. Fischer
- Research on amino acids, plant oils, alkaloids, nucleic acids, etc.
- Reaction kinetics and equilibria, e.g. in hydrolysis and fermentation
Other applications of polarimeters included
- Sugar industries: growing, processing, sugar trade and taxation, uses,...
- Industries where sugar compounds were made from starch
- Production (from plants or synthetically) and quality control of
essential oils and other chemicals such as vanillin and camphor
- Physical chemistry, e.g. A. Werner’s work on coordination compounds
- Compounds with optical activity due to atoms other than carbon
- Medicine, primarily clinical and research uses connected with diabetes
One more class of instruments containing Iceland
spar, was photometers for measuring light intensity
With two Nicol prisms in
tandem, you can attenuate a
light beam by a known ratio
König-Martens
spectrophotometer
Polarization photometers had many functions
- Physics: interactions of light with matter, e.g. reflection and fluorescence
- Chemistry/biochemistry: chemical analysis by absorption or emission
- Astronomy: measurement of the brightness of tens of thousands of stars
- Technology: gloss, dyes, photography, illumination, testing of lamps,....
- H. Wanner’s optical pyrometer, a
modified polarization photometer,
was useful in much high-temperature
research and technical applications
(furnaces) early in the 20th century.
Some areas of progress in physics and materials science before
1930, where Iceland spar was used in experimental situations
- Discovery of M. Faraday’s magneto-optical effect (1845)
- The electro- and magneto-optical effects of J. Kerr (mid-1870s)
- Tests of J.C. Maxwell’s electromagnetic theory; also tests for “aether drift” before
and after appearance of A. Einstein’s theory of relativity (1905)
- Optical properties of metals and of thin films (ellipsometry)
- Participation in research on the Zeeman effect (1896), photoelectric effect, etc.
- Iceland spar was one of the first substances whose anisotropic behavior in
elastic, magnetic, and thermal properties was investigated
...ctd.
Areas of progress........continued
- Photoelastic studies with polarized light, on e.g.
machine- and building-parts subjected to stress
- Photoelastic tests for thermal stresses in glass,
were especially important in the manufacture
of lenses and prisms for microscopes
Sophisticated polariscope
for testing whether lenses
have been properly annealed
(H. Schulz, 1912)
.....ctd.
Areas of progress........continued
- Photoelastic studies with polarized light, on e.g.
machine- and building-parts subjected to stress
- Photoelastic tests for thermal stresses in glass,
were especially important in the manufacture
of lenses and prisms for microscopes
- Extensive research for many decades on the scattering of light by particles
- Iceland spar was employed in studies on ultraviolet radiation
- Chemists used Iceland spar as a reagent e.g. in atomic mass determinations
- Research on colors, color perception, and other aspects of vision
.....ctd.
Areas of progress........continued
Iceland spar crystals (from Iceland and elsewhere) were from 1915 used as a
wavelength standard in X-ray spectroscopy, including many fundamental studies
o
A.H. Compton’s X-ray spectrograph;
C is a diffracting crystal from calcite
Concluding remarks
In this brief presentation, I hope to have shown how Iceland in the
past contributed something to progress in science and technology.
The fortuitous existence in East Iceland of a single “hole in the
ground, miles from nowhere”, as is sometimes said, advanced some
fields of research in natural sciences, such as yours, by tens of years
Iceland spar, in fact, took part in many Nobel-prize discoveries, of
which those by Fischer, Werner and Zeeman have been mentioned
----I wish you a successful meeting and a pleasant stay in Iceland.