Iceland spar in research on colors and vision
Leó Kristjánsson - Sept. 2013
The short compilation below is taken from an updated version of section 29.7 of the
writer's Science Institute Report RH-20-2010. The report can be read or downloaded at this
website. Most of the sources referred to here may be found at the end of the report.
By placing quartz, gypsum or mica plates of different thickness between two Nicol
prisms, one was in a simple way able to produce light having almost any combination of
spectral colors. Complementary color pairs (which together make white light) could also be
produced, and detailed studies on such matters were carried out by Brücke (1848) with a
microscope. Experiments by Dove (1847, 1853), Maxwell (1856) and others also showed that
polarized light was useful in research on color perception. Brücke’s friend H. v. Helmholtz
revived in the 1850s a largely forgotten theory of T. Young regarding the color sensors in the
human eye: these are the cone cells of the retina which are sensitive to light in three
overlapping ranges of the spectrum (see König 1886). Brücke (1866; Ditscheiner 1871) later
described a device with Iceland spar prisms which he called a schistoscope, in a book
intended to teach people in art industries about colors. See the textbook by Rood (1881); the
schistoscope was produced commercially by Steeg & Reuter (1914). Some other papers on
color-vision investigations with the aid of Nicol prisms which have come to my attention are
by E. Rose (1863, 1865), Dobrowolsky (1872, 1876), Raehlmann (1874), Spottiswoode
(1874a), Glan (1881), and Rayleigh (1881b) who continued experiments in this field carried
out by J.C. Maxwell with glass prisms before 1860. An improved device (Chibret 1885)
called chromato-photoptometer was also manufactured commercially for decades (Fig. 1). It
was shaped like a small telescope; by adjusting Nicol prisms and quartz plates inside, some
2700 different colors could be produced for clinical testing of color blindness.
Fig. 1. Left: H.v. Helmholtz' color-mixing apparatus. A person to be tested for color-blindness
looks into the left-hand tube, and compares two colored patches produced from white-light
sources by two Iceland spar prisms on each side. Right: Chibret's chromato-photoptometer.
Further development of equipment containing Nicol prisms and crystal plates for
research on various aspects of color included the leucoscope of König (1882) and the
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chromoscope of Arons (1910, 1912). These instruments were used by many scientists
studying human perception of light and colors (e.g. Brodhun 1888, Schuster 1890, Hering
1890, König 1897, Exner 1902, Lummer 1905, Priest 1920), as well as in the calibration of
some industrial color standards. An anomaloscope designed by Göthlin (1925) for diagnostic
purposes contained three Nicol prisms along with color filters.
Around or before 1878, v. Helmholtz invented vision-testing apparatus where a
researcher could by means of Nicol prisms and other components vary the intensity and color
of two light beams viewed simultaneously by a subject. An improved version was constructed
at the works of Schmidt and Haensch (1893, 1896), one such instrument being presented to
Helmholtz on his 70th birthday by the union of German makers of scientific equipment. König
and Dieterici (1893; Fig. 1) who describe this color-mixing apparatus, made extensive
investigations on the color sensitivity of the eye. It was being produced at least until 1928 and
found use into the 1950s. Other papers on its application include Tonn (1894), Kries and
Nagel (1896), Allen (1902), Engelking (1925), Kravkov (1927) and Hecht and Shlaer (1936)
dealing mostly with various aspects of color blindness, and Kohlrausch (1923) on sensitivity
for changes in light intensity. The Helmholtz instrument was also to some extent used as a
spectrophotometer, such as in Trendelenburg’s (1905) studies on rhodopsin from the eyes of
animals. The Young-Helmholtz theory of color perception, to which J.C. Maxwell made
important contributions, always had great influence in vision research, and it seems to be still
considered valid in many respects. However, much debate took place between followers of
this theory and a rival one proposed by E. Hering whose work was referred to above. The
Young-Helmholtz theory may eventually have absorbed some of Hering’s viewpoints, for
instance regarding how the brain processes nerve signals from the eye.
Numerous other applications for color mixers with Nicol prisms and quartz plates
were found, such as: comparison of light emitted by different lamp types (König 1882),
choice of colors in the manufacture of for instance textiles or decorative objects (Borchardt
1913), development of light filters for the printing and photographic industries (Jones 1914),
and the creation of artificial daylight (Priest 1918). One more field of application concerned
the estimation of high temperatures, as in furnaces, kilns and incinerators (see Nichols 1879,
1880, Crova 1880, Priest 1921, Skinner 1923). A handy telescope device for this purpose
(probably based on Crova's design) was sold by the firm of E. Ducretet (1889; Fig. 2) for
decades (cf. Struers 1925). Burgess and Le Chatelier (1912, p. 348-350) consider this
Ducretet pyrometer to be quite inaccurate, while various authors (McWilliam and Longmuir
1907, Jüptner 1908, Arnold 1917-18) speak favorably of it in connection with for instance
production of steel and coal gas, as well as in the ceramics industry.
Bernard (1856) and Wild (1876) modified their polarizing equipment in order to
produce a color resembling that of the sky, but F. Arago had in fact initiated similar
experiments with more primitive means long before them. Pellin (1899, 1913) advertises a
“chromatometer” invented by L. Andrieu (1886) which was primarily intended for detailed
observations on liquid industrial dyestuffs. It could also be used for monitoring colors of food
pigments, wine, urine, blood etc. Weinschenk (1925, p. 92-94) and Rosenbusch (1924, p. 592-
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593) describe a similar method for studies of mineral colors in thin sections under the
microscope. W. Ostwald (1919) introduced a color mixer nicknamed Pomi (= Polarisationsfarbenmischapparat), containing Nicol and Wollaston prisms. This was in connection with
novel theories of colors on which he and others published much from around 1916 until the
1930s. A sketch of this simple device may be found in Handbuch der Physik 19, p. 683, 1928.
A more advanced color mixer with two Nicol prisms was described by Zeiss (1927). Ostwald
was an admirer of the poet J.W. Goethe and probably adopted some of his views on color
perception. While Ostwald’s ideas in this field were controversial (see e.g. Kohlrausch 1920),
they influenced the standardization of colors for industrial and commercial purposes (cf. entry
on Ostwald in Dictionary of Scientific Biography Suppl. I; Pulfrich 1925, Weigert 1927).
Fig. 2. Left: An optical pyrometer manufactured by Ducretet of Paris for perhaps 40 years.
Right: Müller's (1921) apparatus for testing the light-detection threshold of the human eye.
Spectra could be manipulated in more ways with quartz plates. Thus, Mascart (1874,
p. 396) wanted in some of his experiments to work with only one of the two closely spaced
yellow lines of the sodium spectrum. He found a clever method of eliminating the other one
by destructive interference, by letting his light beam traverse a quartz plate of 3.16 cm
thickness between two Nicol prisms. Fabry and Perot (1900) used the same technique in early
studies on the effects of external conditions on the structure and stability of spectral lines. So
did Voss (1918) when measuring the ratio of the intensities of the sodium lines, and R.W.
Wood (e.g. Wood and Dunoyer 1914, Wood and Mohler 1918) applied it in important
researches on the “resonance radiation” of sodium and some other elements. Attempts were
also made to use the wavelength-dependent rotation in quartz plates cut perpendicular to the
optic axis for spectroscopic purposes (Tait 1880).
Nutting (1913) described an instrument for measuring the intensity and color
composition of light reflected from surfaces, developed at the U.S. Bureau of Standards. It did
not contain quartz plates; instead, four or six Nicol prisms were used to attenuate different
light beams (cf. Fleury 1930). It was sold by A. Hilger, in whose 1924 catalog (Fig. 3) it is
intended for work on papers, powders, liquids and textiles, and for testing color perception.
The ordinary polarization spectrophotometers like those of Glan or König-Martens
often provided service in testing the sensitivity of the eye to differences in color or in
intensity, either on their own or in combination with other instruments (Bohn 1874, Trannin
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1876, Salomonson and Schoute 1904, Kohlrausch 1920). In catalogs around 1900, the
Cambridge Scientific Instrument Co. offers apparatus with Nicol prisms (possibly based on
the Glazebrook spectrophotometer) and a quartz plate, for use in tests for color blindness by
Rayleigh’s (1881) method referred to above. Fedorow and Fedorowa (1929) tested temporary
color-blindness caused by bright lights, with the aid of a Glan instrument. Bezold (1876) and
König and Brodhun (1888) also carried out fundamental research in that field, employing
Nicol prisms and an Iceland spar rhomb. With the projectors designed e.g. by Duboscq, Mach,
Cheshire and Govi for polarized light, certain aspects of vision could be tested such as the
permanence of images in the retina. W. Abney used an Iceland spar double-image prism in a
part of his extensive research on light and color vision (Abney and Festing 1888), also Hering
(1905-20) and Houstoun (1918). Lummer (1909, p. 401-402), Coblentz and Emerson (1918),
Müller (1921; Fig. 2), Hecht and Williams (1922) and others employed a pair of Nicol prisms
for attenuating light beams when studying human light-sensitivity and color perception.
Fig. 3. Left: A Nutting colorimeter patented in 1912 and manufactured by A. Hilger. Right: A
cross-section of Salomonson's (1919) ophthalmoscope, with two Iceland spar prisms. A rightangle prism illuminates the patient's eye with polarized light, to reduce unwanted reflections.
Various material properties of the tissues of the eye were investigated with the aid of
calcite plates (Carion 1853) and later with instruments containing Nicol prisms, for instance
by Nordenson (1921). Ophthalmometers based on double refraction in prisms of quartz or
Iceland spar were also invented. One such instrument (Javal and Schiötz 1881) was acclaimed
as a significant achievement in research on astigmatism. Improved versions were on the
market into the 1930s at least, and some related designs were patented. Some
ophthalmoscopes for examining the interior of the eye employed Nicol prisms to eliminate
disturbing reflections, as had been done with mirrors in equipment introduced by Helmholtz
(1851). See the paper by Salomonson (1919, Fig. 3), a patent obtained by Keeler (1936), and
especially the publications of Koeppe (1921, etc.) who studied eye defects with polarized light
from a lamp invented by A. Gullstrand. Gullstrand (1906) himself employed Nicol prisms in
some of his ophthalmological work for which he received a Nobel prize in 1911.
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