Iceland spar and its early applications in microscopes and other optical equipment

1. 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)

2. Visitors to Iceland perhaps ask themselves... ...did this remote and sparsely populated island ever contribute anything significant to the outside world ? Yes, at least inone 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)

3. 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

4. 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

5. 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.

6. 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

7. Iceland spar and optics, early 19th century • E.L. Malus discovered in 1808 that reflection from smooth non-metallic 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

8. Brewster realized that some crystals have one axis of symmetry for light propagation, others have two such axesThe interference figures appearing in crystal plates in convergent polarized light have been valuable in the identification of crystals Uniaxial crystal Biaxial crystal

9. 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

10. 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

11. 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

12. 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.

13. 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

14. Microscopes for the study of crystals developed gradually from the 1830sThe type on the right was in use around1860, for instance by A. Des Cloizeaux  Nicol prism Powell 1841

15. 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 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

16. A main reason for the use of polarized light in microscopy is that differences in the birefringence of minerals appear as different colorsThis was the discovery made by Arago in 1811 F. Zirkel Gabbro. Top: ordinary light, bottom: crossed nicols

17. Polarizing microscopes, made commercially for the study of thin sections in parallel or convergent light, appeared in the 1870’s

18. 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

19. Specialized instruments for the study of individual mineral grains were designed, as well as teaching equipment Projector, 1875 Axial-angle apparatus, ca. 1880

20. 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 difficultThis caused a “spar famine” among many users for decades, and led to attempts at finding other mines or substitutesFrom 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 GermanySites in South Africa became the chief supplier after 1920

21. In spite of the “spar famine”, the development of petrographic microscopes continued... Swift/Dick 1889 R. Fuess ca. 1900 E. Leitz ca. 1905 Bausch & Lomb ca. 1910 A. Nachet 1900-10 C. Reichert 1909

22. .....and continued Photo: D. Kile Zeiss/Winkel 1918, universal stage Leitz 1919, with reflected-light and other options

23. 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)

24. Another important application of Nicol prisms from Iceland spar was in polarimeters, which measure optical activityIt 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.

25. 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)

26. 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

27. 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

28. 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

29. 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.

30. 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.

31. 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.

32. 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.

33. 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

34. 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.