Glimpses from

the Early History

of Petrographic

Microscopes

Leo Kristjansson

Institute of Earth

Sciences,

University of Iceland

Talk at the Spring

Meeting of the

Geoscience Soc.

of Iceland, 2015

Photos from D. Kile: The

Petrographic Microscope

E. Leitz

1896Bausch &

Lomb 1902

E.L. Malus announced in 1809 that ordinary light changed in a certain

way by reflection from smooth surfaces. This change which he termed

“polarization”, also occurs when a light ray passes through crystals.

The discovery by Malus created much interest in optics. The crystals he used

came from Helgustadir, East Iceland. They were first described in 1669, and

this variety of calcite has been known as Iceland spar since around 1780.

Simplified diagram,

based on later

observations

Fr. Arago noted in 1811 that polarized white light created colors in transparent

crystalline substances, in the same way as colors appear when unpolarized light

is reflected from oil slicks and soap bubbles.

Colors in a

6 cm wide

piece of mica,

of varying

thickness

Equipment for studying colors in crystalline plates

(without magnification) was invented already in 1812

and later produced commercially. A device introduced

by J. Nörrenberg in 1833, was popular for decades.

Glass mirrors are employed here, the lower one is

called the polarizer, the upper one is the analyzer.

J.B. Biot found in 1815 that thin sheets of tourmaline

can also be used for polarizing light.

In the same year, D. Brewster described color fringes

that appear in crystal plates in convergent polarized light:

The patterns showed that in three classes (systems) of crystals there was a single

direction along which the two perpendicularly polarized rays travelled at equal speed.

In three other systems, two directions had this property. The angle between these

directions was characteristic for each biaxial mineral.

This became an essential

tool for identifying minerals.

Projectors were sold, to

display such patterns.

Geologists took rather a long time to appreciate the usefulness of Arago’s color

phenomena which had subsequently been studied in detail by Biot.

The first one to make a polarized-light microscope

was G.B. Amici around 1832.

It had a glass-mirror polarizer and a piece

of Iceland spar above the ocular.

In 1829, W. Nicol invented prisms of Iceland

spar which delivered only one polarized ray.

They were first used in a microscope by H. Fox

Talbot in 1834, soon becoming common as optional

accessories for biologists and amateurs.

Book, 1855

1857

Around 1851, A. Oschatz and H.C. Sorby

started making thin sections of rocks to

study in microscopes. Sorby wrote many

papers on this topic, emphasizing the use

of polarized light. He also designed a

spectroscope for use with microscopes.

Initially, only few followed their example.

Microscopes containing 1 or 2 Nicol prisms

(replacing mirrors and tourmaline) and 2 to

9 lenses appeared around 1860. These

enabled more detailed studies of the color

phenomena seen in crystal plates. The one

on the right was used by J. Grailich in 1858.

The next slide shows similar instruments,

one belonging to A. Des Cloizeaux who

published extensive works on mineralogy.

He travelled in Iceland in 1845 and 1846.

Left: a Nörrenberg microscope from 1858. Right: the microscope of Des Cloizeaux 1864, with

a mirror polarizer. Both are intended for studies of crystal plates, and contain several lenses.

H.C. Sorby (from Sheffield) travelled in Germany 1861. There he met the young

mineralogist F. Zirkel, whose doctoral dissertation dealt with the geology and

petrography of Iceland following a visit to this country in 1860.

Zirkel immediately realized the potential offered by thin sections for geological

research. In a paper in 1863 (partly on Icelandic material) he described methods

for making these, continuing in the following years to stress the advantages of

polarized light. Also H. Vogelsang, G. Tschermak, H. Rosenbusch and P. Groth

In the 1860s, later joined by A. Michel-Lévy, F. Fouqué, É. Bertrand and others.

Zirkel 1873

One of the first micro-

scopes with built-in

Nicol prisms was con-

structed by R. Fuess for

P. Groth in 1871 (left).

Fuess made another one

for H. Rosenbusch 1876.

The Fuess workshop

became famous for its

designs of many petro-

graphical instruments.

In 1867-69 the U.S. Congress sponsored a comprehensive scientific study

on a cross-section of the country, along the 40th parallel. F. Zirkel carried

out the petrographical part of this project, his large report appearing in 1876.

It contained large color illustrations of 44 thin sections, which received much

attention. Many geologists became interested in making use of this technique.

Increasingly, microscope producers offered models containing Nicol prisms,

dedicated to petrography. The analyzer prism was moved to below the ocular.

A. Nachet,

Paris 1876W. Watson,

London 1879

Various steps of progress in the petrological sciences, effected by the study

of thin sections of rocks with polarized light:

-Similar-looking minerals in a rock specimen could be distinguished

-Improved knowledge of what composed the so-called “groundmass”

-Many minerals previously thought to be rare, were in fact common

-It was seen that some minerals (including ores) tended to occur together

-Crystal structure features (zoning, twinning, deformation etc.) became visible

-The order in which minerals had been formed in a rock, was clarified

-Complex solid-solution series like the feldspars were sorted out

-The chemical composition of many minerals could be estimated optically

-Classifications were revised, with new concepts (petrographic provinces, &c)

The upper photo shows a

thin section (0.03 mm) of

gabbro c. 1 cm wide in

ordinary transmitted light.

The lower photo shows the

same section with “crossed

nicols”, i.e. in polarized light.

From the book “A Colour Atlas

of Rocks and Minerals in Thin

Section” by W.S. MacKenzie

and A.E. Adams, 1994.

New developments in geosciences and elsewhere aided by the use of

polarizing microscopes in 1880-1920 :

-Production of many minerals, rock types and even meteorites in furnaces

-Understanding of the generation of secondary minerals from primary ones

-Heating of mineral assemblages up to 1500°C: phase diagrams, equilibria

-Research on many industrial solids: abrasives, high-temperature quartz,

ceramics, refractories, cements and other ingredients in concrete,...

-Investigations on “liquid crystals” where polarized light was indispensable

-Prospecting for mineral resources, and their exploitation

-Characterization of new natural or man-made crystalline compounds

-Microscopes of this type were also useful in biological research, e.g. on

the histology of bones, muscles, nerves and shells, and on textile fibers

Novelties after 1920 included reflected-light microscopy of metals and ores

R. Fuess

1891

C. Zeiss

1891

Various specialized features appeared around 1900, including heated stages to

study crystallization processes (left), and universal-stage microscopes

Advanced equipment for studying single

crystals included axial-angle meters,

refractometers, and totalreflectometers

Available accessories included the Michel-

Lévy color comparator (right), various phase-

compensating devices, and cap nicols.

There was also an abundance of equipment

for teaching and demonstration purposes,

such as the projector below.

Concluding remarks:

This has been a brief resumé of the history of microscopical mineralogy and

petrography on polarized light, also mentioning related developments in

crystallography.

Nicol prisms were an essential component of these microscopes from c.1870

onwards. Most of the Iceland spar needed for their construction came from

Helgustadir in East Iceland, until 1920 when mines were found in SW-Africa.

That period encompasses what has been called a “golden age” in this field

of microscopical research. The use of polarized light led to great progress in

geology, also leaving its mark in biology and in materials science.

This account however covers only a fragment of the achievements made

possible by the availability of large transparent crystals of Iceland spar.

See more at https://notendur.hi.is/~leo

Geoscience Soc.

of Iceland, Spring

Meeting 2015

The photographs of microscopes are from the book “The Petrographic Microscope” issued as

Special Publication Number One, Supplement to “The Mineralogical Record”, Nov.-Dec. 2003

Nachet

1910Winkel

1918