Applications of Iceland spar prisms in observational astronomy, 1860-1930

1. Applications of Iceland spar prisms in observational astronomy, 1860-1930 Leó Kristjánsson Institute of Earth Sciences

2. 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.

3. 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.

4. 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.

5. 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

6. 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 Star spectrum with three visible H-lines and several uv ones Sir William Huggins

7. 2. Photometryof 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.

8. Zöllner photometer

9. 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, wedge-type 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

10. 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.

11. Ch. Nordmann’s telescope and “photomètre héterochrome”, with Iceland spar prisms, used c. 1910 to estimate star temperatures

12. 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 spectrophoto- meter, 1877 Corona, 1896

13. 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

14. 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

15. 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

16. So, more instruments can be described, but I’ll stop here – Thank you