Applications of Iceland spar prisms in
observational astronomy, 1860-1930
Leó Kristjánsson
Institute of Earth Sciences
A few of us here in the Faculty of Science happen to
be interested in the history of scientific research. My
own interests chiefly concern results acquired in
Iceland by foreign expeditions.
One of my projects has to do with the properties of
Iceland spar crystals and their impact upon the
natural sciences, mostly in the 19th century and until
the 1920’s.
Iceland spar (silfurberg) is a variety of the common
mineral calcite (trigonal CaCO3). It exhibits strong
anisotropy, manifested e.g. in double refraction.
Large clear crystals of Iceland spar were mined and exported
intermittently at Helgustaðir, Reyðarfjörður until 1925.
Comparable material was only found in limited amounts
elsewhere before 1900; in significant amounts from 1920.
The conclusion from my research is that this activity
constituted Iceland’s most important contribution to the
outside world in the last millennium.
Copies of a recent report on the uses of Iceland spar are
available from me, also reprints of papers and crystal
specimens.
This talk only deals with a small fraction of the story.
Fields where Iceland spar has contributed
indirectly to astronomical research
• The nature of light and other electromagnetic radiation
• Interaction of light and matter: absorption and emission of
light by gases, luminescence, scattering, dispersion,...
• Influence on the development of modern physics (quantum
mechanics, relativity, spectroscopy, X-rays,...)
• Manufacture of strain-free glass
• Materials science; properties of matter at high temperatures
• I am not covering these aspects here.
• Instead, I will talk about the diverse direct applications of
Iceland spar crystals in astronomical observations.
1. Ultraviolet spectroscopy of stars
Ordinary glass is relatively opaque to ultraviolet rays. Therefore for instance,
scientists only knew the four visible spectral lines of hot hydrogen before 1876.
No one understood the laws governing spectral line emissions.
Sun
Hydrogen
Mercury vapor
In 1876-80, W. Huggins published results on the spectra of stars, including
some bluish ones like Sirius. He used a dispersion prism of Iceland spar and
lenses of quartz. His spectra included several lines in the near ultraviolet.
Their wavelengths formed a regular progression with the visible H lines.
This was a breakthrough in the study of spectra, leading to the Balmer
formula for hydrogen lines and many subsequent discoveries of regularity.
Huggins’ work also was a step towards the classification of stars by their
physical properties. For more details, see my paper in Verpill 2007.
I. spar
Sir William Huggins
Star spectrum with
three visible H-lines
and several uv ones
2. Photometry of stars
Before 1860, estimates of the brightness (magnitude) of stars were mostly
done by eye.
Then, Fr. Zöllner invented a photometer with two Iceland spar prisms
where the intensity of light from a star could be measured quantitatively by
comparison with the flame of a standard lamp.
After improvements, tens of Zöllner photometers were
installed in observatories. Some were in use until 1930.
The major effort with these meters (Photometrische
Durchmusterung des nördlichen Himmels) was carried
out in Potsdam in 1886-1905 and included some 19000
stars down to magnitude 7.5.
Zöllner photometer
Zöllner’s photometer was improved upon from 1876
by E.C. Pickering in the Harvard College Observatory.
He used the Pole Star as standard, instead of a lamp.
Many tens of thousands of stars were measured with his instruments
In the north and south hemispheres in 1879-1906. Later on, wedgetype photometers, photographic methods and photocells took over.
Very detailed measurements were also made on many variable
stars, both periodic ones and novae. These and other studies
enabled e.g. a classification of the periodic stars into eclipsing
binaries and pulsating stars, and gave information on their sizes etc.
One of Pickering’s photometers
3. Color and temperature estimates
By inserting glass dispersion prisms, quartz plates, or color filters in the
light path of a Nicol-prism photometer, the color of a star could be estimated.
Assuming black-body radiation, an effective temperature could be derived.
Another simple method of sorting star spectra was based on the different
sensitivities of visual and photographic photometers at short wavelengths.
Equipment with Iceland spar prisms was involved in quantitative work in this
field early in the 20th century. The spectral types were later employed in the
classification of stars according to mass, age etc.
Ch. Nordmann’s
telescope and
“photomètre
héterochrome”,
with Iceland spar
prisms, used c.
1910 to estimate
star temperatures
4. Observations on the solar system
Early in the 19th century, observations on the nature of
sunlight with Iceland spar prisms indicated that the Sun
was gaseous. Later, observations of the solar corona
during eclipses e.g. with polariscopes from Iceland spar
were used in studies of the nature of its light emission.
Ultraviolet observations were also made with spar prisms.
H.C. Vogel’s
spectrophotometer, 1877
Corona, 1896
Extensive measurements on the intensity and vibration state of sunlight
reflected by the Moon, planets, satellites, minor planets and comets were
carried out in 1860-1930 with instruments incorporating Nicol prisms.
These properties could be compared to those of light reflected from
various types of rocks, fragmented materials, dust, ice etc.
B. Lyot
5. Magnetic fields in the Sun
The Zeeman effect (1896): if an emitter of
light is in a strong magnetic field, then the
frequency of that light changes by an amount
proportional to the field strength. Its vibration
direction also changes, depending on that of
the field relative to the emission direction.
G.E. Hale
This was used from 1908 by G.E. Hale in
California to map strong magnetic fields in
sunspots, with the aid of Nicol prisms from
Iceland spar. It was a major discovery.
Hale and collaborators estimated the main
magnetic field of the Sun in 1913, and
demonstrated that the polarity of this field
reverses between 11-year sunspot cycles.
Pair of N and S sunspots, 1908
6. Some other applications of Iceland spar
• Micrometric measurements of e.g.
diameters of the planets and distances
between double stars. These were
initiated before 1800 but rarely used;
revived in 1949 by B. Lyot
• Calibration of gray glass-wedges
for use in photometric work
• Calibration and measurement of
darkening in photographic photometry
• Studies of atmospheric light absorption
(by ozone, etc.)
• Birefringent filters invented by B. Lyot 1933 for isolating very narrow
(2 Å) spectral portions in images of the Sun
So, more instruments can be described, but I’ll stop here – Thank you