Humboldt State University ® Department of Chemistry
The article below appeared in Bulletin of the Scientific Instrument Society 62 (September1999) pp 19-22.
Nearly every student of organic chemistry has used a refractometer to aid in the identification of compounds. Today it is predominantly a tool for quality control or quickly checking on compound identity. It has not always been thus.
The refractometer measures the real part of the refractive index and thus helps to answer three different types of questions. First, and most simply, it is useful in the empirical identification of pure substances, it can act as a criterion of purity, and it serves in the quantitative analysis of solutions. These characterizations are made possible by the precision and accuracy of refractometers. Second, the evaluation of dipole moments of substances via measures of dielectric constant at a single temperature requires the knowledge of their refractive indexes. Third, refractive index measured as a function of wavelength, in concert with measurement of molar absorptivity characterize the optical properties of a given molecule. These measures in turn provide information on the electronic structures of molecules. As an example, refractometry can be useful in the determination of chain length and isomerism in organic molecules.1 The development of modern NMR and mass spectrometers have largely displaced the use of refractometry in such studies, giving less ambiguous answers regarding molecular structures, but at a great increase in instrumental complexity and cost.
A variety of other instruments have been derived from the original Abbe design. Carl Zeiss manufactured a number of specialty instruments with restricted refractive index ranges for fats, sugar, clinical studies etc. These instruments differed from the standard Abbe only in range, ease of operation, lower cost, etc. All of their measurements could generally be accomplished with a standard Abbe refractometer. Some of these instruments are described by Tilton and Taylor.2
The modern Abbe refractometer invented at the Carl Zeiss Works was exclusively manufactured by Zeiss until the early twentieth century. The explosive growth of laboratory work after WW 1 led a number of other companies to begin its manufacture as well, including Adam Hilgar and Stanley in Great Britain, and Spencer Lens Co., Bausch & Lomb, Gaertner, and Valentine in the U. S.
Ernst Abbe constructed the first "Abbe"3 refractometer in 1869.4 Five years later, in 1874, he published a comprehensive booklet.5 In it he discusses the theory and described
1 Detailed treatments of refractometry
and the information obtainable using refractometry are provided
by: Bauer, N., K. Fajans, and S. Z. Lewin, "Refractometry,"
in Physical Methods of Organic Chemistry, 3rd ed.
Vol I part two (Arnold Weissberger, ed.) Interscience Publishers,
Inc., New York (1960) pp 1139-1281; and Tilton, Leroy W. and John
K.Taylor, "Refractive Index Measurement," in Physical
Methods in Chemical Analysis, 2nd ed. Vol I (Walter G. Berl,
ed.) Academic Press, New York (1960) pp 412-462.
2 Ibid, pp 431-2.
3 Instrument manufactureres, including Carl Zeiss, frequently describe Abbe refractometers using the term Abbe without the diacritical mark. I will use the correct spelling when describing the instrument, but I defer to the manufacturers spellings in describing their individual instruments.
4 Wittig, Joachim. Earnst Abbe. BSB B. G. Teubner Verlagsgesellschaft, Leipzig (1989) pg 60.
5 Abbe, E. Neue Apparate zur Bestimmung des Brechungs - und Zerstreuungsvermögens fester und fluüssiger Körper. Mauke's Verlag, Jena (1874) Taffel: Fig. 5-7.
instruments for the measurement of refractive index using prisms and by total reflection. It is here that he first describes and illustrates the Abbe refractometer for determining the refractive index of fluids:
This initial instrument includes Amici prisms and has all of the essential features of a modern Abbe refractometer, though without temperature jacketing.
The Manufacture of Abbe Refractometers by Carl Zeiss
Beginning around 1881 refractometers were described and offered, though not illustrated, in Carl Zeiss microscope catalogs.6 For example, the No. 27 catalog of 1885 lists four instruments including a "No. 75 Grosses Refractometer nach Abbe," corresponding to Abbe's original design.7 In 1890 the Instrument Department of the Carl Zeiss Works was formed under the direction of Carl Pulfrich.8 Shortly thereafter Carl Zeiss published their catalog, Optical Measuring Instruments (1893).9 In this catalog it is stated in a preface that: "Until recently these instruments have only occasionally been supplied for the use of others [than the Zeiss Works]. But ..., we have now formed a special department in which instruments in this class are regularly manufactured."
A similar instrument to that originally described by Abbe in 1874 is detailed and first illustrated by Zeiss in the 1893 catalog:
6 Personnal communication (1998),
Dr. Helga Beez, Carl Zeiss Optical Museum, Jena.
7 Carl Zeiss Optical Works. No. 27 Mikroscope und mikroscopische Hilfsapparate der Optichen WQerkstätte von Carl Zeiss. Jena (1885) pp 47-48.
8 Auerbach, Felix. The Zeiss Works and the Carl Zeiss Foundation in Jena. (English translation by R Kanthack) W. & G. Foyle, Ltd, London (1925).
9 Carl Zeiss Optical Works. Optical Meauring Instruments. Carl Zeiss Optical Works, Jena (1893) pp 9, 10.
The illustration closely matches refractometers with serial numbers No. 148 held at the Smithsonian Institution10 and No. 162 held in this author's collection,11 respectively:
(The catalog notes that the magnifier lens seen on the alidade on these instruments is not shown in the catalog illustration).
Of these two instruments, No. 148 is finished in lacquered (telescope and base) and black oxidized brass, while No. 162 was originally finished in nickel plate (telescope and base) and black oxidized brass. No. 162 also includes its original fitted case, also numbered 162:
It is interesting that an apparent difference between these instruments and the illustration in the 1893 catalog, the cursor on the alidade and the scale, is shown in Abbe's original 1874 illustration as an alternate arrangement:
10 "No. 148 was delivered to
J. Queen & Co. in Philadelphia on July, 2., 1890." Personal
communication (1998), Dr. Wolfgang Wimmer, Archivar, Carl Zeiss
11 "No. 162 was delivered to Schimmel & Co. in Leipzig on November, 11., 1891." Personal communication (1998), Dr. Wolfgang Wimmer, Archivar, Carl Zeiss Jena GMBH.
The 1893 Optical Measuring Instruments catalog also describes, but does not illustrate, an Abbe Refractometer with temperature jacketed prisms. The specifications given in the catalog for these instruments include a refractive index range of 1.30-1.70, reading directly in the third decimal place, with estimation to 2 units in the fourth decimal place. The same specifications are given for the Carl Zeiss Abbe, the Bausch & Lomb Abbe 3-L, and the American Optical Digital Abbe refractometers in 1981.12
The subsequent evolution of the Abbe refractometer involves modifications to increase the convenience, ease-of-use, and/or stability of the instrument, and, later, development of instruments for operators as opposed to professional users. Some of these changes are traced below.
A number of slight modifications are seen in subsequent Carl Zeiss catalog illustrations beginning in 189913 or before, and extending to at least 1910.14 Thus the alidade and scale appear as on the actual instruments No 148 and 162 above. In addition the pillar is reversed relative to these instruments and the 1893 illustrations. The mirror is now larger and rectangular instead of round, as seen in subsequent illustrations through 1950, providing better illumination of the prisms. Both un-jacketed prism:
and jacketed prism instruments:
are shown in Zeiss catalogs of this era.15
12 Fisher Scientific Co. Catalog.
Fisher Scientific Company, Pittsburg (1981) pg 1031-2.
13 Carl Zeiss Optiche Werkstaette. Specialkatalog über Spectrometer und Refractometer für feste und flüssige körper; hilfsapparate. Carl Zeiss Optiche Werkstaette, Jena (1899) pp 39-43.
14 Eimer & Amend. Illustrated Catalog of Chemical Apparatus, Assay Goods and Laboratory Supplies. Eimer & Amend, New York (1910) p 323.
15 Zeiss. Abbe's Refractometers. 3rd Edition (Mess. 172). Carl Zeiss, Jena (1907) pp 2 & 9.
A new design with a round, cast iron base, appears in a catalog of Carl Zeiss instruments exhibited in Dresden in 1911:16
The instrument is heavier than the previous design, and no longer folds up for compact storage. It is now loaded in the normal operating position instead of the folded position of the earlier designs. The telescope and scale are adjustable for viewing angle as well. Overall the instrument is better designed for routine use in the lab. An example of this instrument, serial number 15645, circa 192017 is held in this authors collection. Note the variation in the thermometer holder in the photograph (probably a shop-made custom adaptor) versus the catalog engraving:
16 Carl Zeiss Optical Works. Catalogue
to the Collection of Optical Instruments Exhibited by the Carl
Zeiss Optical Works Jena. Dresden International Hygiene Exhibition
(1911) pg 13.
17 "No. 15645 was produced before February, 19., 1920 and delivered to Sussfield, Lorsch & Co. in New York on February 27 1920." Personal communication (1998), Dr. Wolfgang Wimmer, Archivar, Carl Zeiss Jena GMBH.
Although the instrument above is illustrated in catalogs as late as 1927,18 a new model is illustrated in Zeiss' Directions for using the Abbe Refractometer published in 1926.19 The new design, modified from the 1911 model, is shown below as illustrated in a 1928 Fisher catalog.20 The mirror is now placed on a swinging arm attached to the prism axis (to provide better illumination) and a rack and pinion adjustment eases the adjustment of the alidade:
Zeiss introduced a new, quite different, design for the Abbe refractometer in 1950,21 which continues to appear in vendor catalogs through at least the 1980's.
The Manufacture of Abbe Refractometers by Makers Other then Zeiss
Until the end of WW I Carl Zeiss appears to have had a monopoly in the production of Abbe refractometers. However, following the war other manufacturers introduced their own versions of the Abbe refractometer along with other scientific instrumentation.22 This sudden interest by other firms in scientific instrumentation seems to have been partially a result of instrument companies trying to maintain their businesses while converting from a wartime footing. Simultaneously however, an increased need for instrumentation arose as industries became more interested in research, and went to a more scientific basis for production.23
18 Cenco. Laboratory Apparatus
for Chemical, Industrial, Metallurgical, Bacteriological, Board
of Health, Clinical, Hospital and Commercial Testing Laboratories.
Catalog C. Central Scientific Company, Chicago (1927) pg 592.
The engraving in this catalog appears to be identical to the one
used in 1911 in reference 10, above.
19 Zeiss. Directions for using the Abbe Refractometer. 4th Edition (Mess. 172). Carl Zeiss, Jena (1926) pp 2 & 7.
20 Fisher Scientific Co. Laboratory Apparatus and Reagents for Chemical, Metalurgical and Biological Laboratories. Fisher Scientific Company, Pittsburg (1928) pg 508.
21 Internal document, Reifezeit für High-Tech, Carl Zeiss Jena (c 1990)
22 The firm of Carl Zeiss provides another rationale for the sudden production of refractometers in Britain and the US. Thus in their internal document Entwurf zu einer Chronik der Abteilung Mess (An Outline of the History of the Measurement Division) is the following entry: 1919 Die amerikanische Firma Bausch & Lomb Rochester (USA) und die Firma Adam Hilger in London bauen nach den ihr durch den Krigesausgang übereigneten Pateten Zeiss-Geräte mit den Zeiss' schen Originalbezeicnumgen, z.B.: Refractometer nach Abbe; Refraktometer nach Pulfrich; Eintauchrefraktometer. (As a result of patents that were transferred because of the outcome of the war, the American firm Bausch & Lomb (Rochester, USA) and the firm of Adam Hilger in London were able to produce instruments using original Zeiss [specifications/plans/designs], e.g.
The Abbe Refractometer
The Pulfrich Refractometer
The Immersion Refractometer) [trans. courtesy B. Codispotti]
23 Williams, Mari E. W. The Precision
Makers: A history of the instrument industry in Britain and France,
1870-1939. Routledge, London(1994) pg 145.
In Britain for example, chemical, oil, and pharmaceutical companies began to build up their in-house research laboratories.24 At the same time chemistry as a field was becoming more dependent on instrumental measures as physical methods became more accepted. The net result was a greatly expanded market for instrumentation generally, including refractometers, in both the industrial and the academic sector.
In the U.S. two companies began manufacturing refractometers in the early 1920's: Bausch & Lomb Optical Co. and Spencer Lens Company.25 Bausch & Lomb's initial design is nearly identical in specifications and even appearance to the Zeiss instrument illustrated in 1911, as seen in the illustration from Braun's 1922 catalog:26
By 1926 B & L added a slow motion screw fine adjustment to the alidade as shown in a 1926 brochure:27
The instrument appears otherwise to be essentially identical to the original model. This design remained apparently unchanged until the introduction of the "Abbe 56" refractometer with enclosed case and optically illuminated glass scales around 1950:28
25 Other American firms manufacturing Abbe refractometers later in the century included Gaertner Scientific Corp. and Valentine Technical Instrument Corp. Neither will be covered in this survey.
26 Braun Corporation. Catalog No.8, Laboratory Instruments Apparatus and Supplies. The Braun Corporation, Los Angeles (1922) pg 410.
27 Bausch & Lomb. Microscopes, Microtomes, Colorimeters, Optical Measuring Instruments and Accessories. Bausch & Lomb Optical Company, New York (1926) pg 171.
28 The Abbe 56 is illustrated in both the 1950 catalogs of Adolf Frese and Cenco.
The more refined and self-contained "Abbe 3L" instrument followed in 1956:29
In this instrument the illuminated optical refractive index scale and the cross-hairs and boardeline are viewed alternately through a single eyepiece. Illumination is via a built-in source. The scales are enclosed and protected from the environment. This convenient and very popular instrument has continued in production, with slight modifications, to the present day.30
Spencer's original instrument, apparently introduced in 1920,31 also owes much to Zeiss' 1911 style refractometer. However Spencer substituted a tripod base and innovated a gear drive for the alidade, as shown in the 1922 Cenco catalog illustration below:32
By 1924, as illustrated in a company brochure, Spencer had redesigned the base to a triangular form and placed the mirror on a slide for better light adjustment:
This design remained essentially unchanged through the mid-1970's, with the exception that sometime before 1940 the gear drive for the alidade had been replaced by a slow motion screw similar to Bausch & Lomb's, as may be seen in this 1953 specimen:
29 The Bausch & Lomb manual
for this instrument, Bausch & Lomb Abbe-3L Refractometer
Reference Manual, has the code VIII-56 on the back cover,
which experience has led me to believe indicates a publication
date of August 1956. The Abbe-3L is also illustrated as a related
instrument in the Bausch & Lomb Spectrograph brochure
dated Dec. 1956. Finally, the 1955 Chemical Rubber Company Catalog
A-L still advertizes the Abbe 56 as B&L's refractometer
(personal communication, Bill Burns, Long Island NY).
30 The Abbe 3L refractometer is currently sold by the Spectronic Instrument Co., successor to Milton-Roy Co., which purchased the Analytical Products Division of Bausch & Lomb Optical Co. in 1985.
31 The Spencer Lens Co. Catalog of Spencer Microscopes Microtonmes and Accessories (1920) copyright 1919 states "We have in process of development a line of Spectrometers, Spectroscopes, Polariscopes and Refractometers." An insert found in a copy of this catalog in the National Museum of American History (Smithsonian Institute) Library describes and illustrates Spencer's origianl refractometer with tripod base.
32 Cenco. Laboratory Apparatus for Chemical, ... Catalog C. Central Scientific Company, Chicago (1922) pg 427.
Around 1976 American Optical Co. (successor to Spencer) introduced an Abbe refractometer with digital readout of the refractive index. This easy to read instrument is still sold under the Leica name.33
In Britain the firm of Adam Hilgar introduced an Abbe refractometer by late 1918.34 As illustrated in their brochures this instrumented is patterned after the Carl Zeiss Abbe refractometers of 1899-1910:35
The 1952 Hilger catalogue shows an instrument still obviously based on Zeiss models, but more similar to the Zeiss refractometer of 1924-1950:36
The Hilgar designs thus appear to be very conservative interpretations of their Zeiss predecessors.
In over a century of production Abbes original refractometer has not been improved upon in terms of accuracy, precision, or range. Rather, the story of this once key research instrument, which remains irreplaceable in the modern quality control and analysis laboratory, is a story of modification for convenience, cost, and the transition from practitioners to operators as its major clients.
I would like to thank Debbie Warner of the U.S. National Museum of American History for providing me access to the refractometer in their collection, for introducing me to their library, and for providing contacts for additional research; Dr. Helga Beez of the Carl Zeiss Optical Museum in Jena for providing information on the Abbe refractometer from their archives, and Dr. Wolfgang Wimmer, Archivar at Carl Zeiss Jena GMBH, for providing reprints of various internal and published documents on the refractometer, and for providing data on individual instruments from the Zeiss archives; Dr. William Golden of Humboldt State University for encouragement, and for reading drafts and for suggesting both stylistic and substantive changes in the document. I would also like to thank Dr. Kathleen LaBahn of Humboldt State University for aid in translating passages of Abbes original German article (1874) and Burt Codispoti, also of Humboldt State University, for his enthusiastic aid in translating various Zeiss documents from the original German.
33 American Optical Co. sold its
instrument group to Cambridge Instrument Co. in 1986. Subsequent
mergers, acquisitions etc., outlined on Leica's web page, brought
this refractometer under Leica's umbrella.
34 Adam Hilger Limited. Abbe Refractometer with Water Jacketed Prisms. Adam Hilger Limited, London (October 1, 1918). Abbe refractometers are not shown in Hilger's 1914 general catalogue
35 General Catalogue of the Manufactures of Adam Hilger, Ltd. Adam Hilger Limited, London (October 1924) pg M 14.
36 Hilger Catalog M. Hilger & Watts Ltd., Hilger Division, London (November 1952) pg M 2.