Humboldt State University

Robert A. Paselk Scientific Instrument Museum

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Precision Refractometer

Bausch & Lomb

Ser No: 00268Z


Private collection

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The Precision Refractometer from B&L is designed to give an order of magnitude greater precision than the standard Abbe Refractometer. It is a "fifth decimal" instrument rather than a "fourth decimal" instrument. Basically it is an Abbe Refractometer turned on its side. The instrument can thus accommodate a heavy, vertical, lapped bearing like those characteristic of high precision instruments such as transits and spectrometers, eliminating the mechanical sloppiness of the classic instrument. Additional design features to increase precision include an increased radius of the scale sector enabling readings good to a few seconds of arc, or 3 units in the fifth decimal of refractive index. (The article describing this instrument claimed the dividing engine used for graduating the scale must be better than 2 seconds of arc.) The telescope is based on that used in immersion refractometers (which are good to 3 units in the fifth decimal place) with its higher power and longer focal length. The use of a built-in sodium light source allows the Amici compensating prisms to be eliminated, avoiding the inevitable degradation of precision due to the more complex optical path and alignment errors. Of course the temperature of the prisms and specimen must be maintained to better than 0.1°C to achieve "fifth decimal" precision. The catalog scan is taken from Eimer & Amend/Fisher Scientific Company Modern Laboratory Appliances90.Pittsburg/New York (1942, reprinted 1946).


"The increasing use of refractometry in the laboratory and in process control, and the current demand for greater accuracy [in the sugar industry]...." were given a reasons for the development of this instrument by Bausch & Lomb scientists in 1938.2 The design of this refractometer is based on a careful analysis of the critical angle refractometers commonly in use (Abbé, Pulfrich, and immersion or dipping) at that time, and a careful consideration of instrumental design. The resulting refractometer attempts to combine the best features of the Abbé design (versatility, convenience, speed and ease of use) and the immersion design (high precision).2, 3 Though designed for an absolute accuracy of 2 or 3 in the fifth place, Bauer and Lewin state that the instrument appears to be accurate to only 6 in the fifth place in terms of reproducibility etc. As a comparison they also note that the Precision Refractometer was found to have a sensitivity "about twice as good" as the Zeiss sugar refractometer.4 Though designed for the sugar industry, its precision attracted other users. I first saw a B&L Precision Refractometer in a biochemistry research lab, where it was used in the creation of high quality sucrose gradients for ultracentrifugation experiments.

A brief essay, The Chemical Refractometer, describes the characteristics, design, and use of various refractometers. A detailed history, The Evolution of the Abbé Refractometer, traces the development of the predecessor to this more specialized instrument.


The instrument stands 12 1/2" high at its highest point (top of eyepiece) and is 15" long exclusive of the lamp and lamp arm. The instrument is finished in black japanned enamel except for the lamp housing which is painted in black crinkle enamel. The enclosed surfaces of the prism housing, the circular catchment around the prism housing and the interior surface of the drain trough have a heavy polished nickel(?) plating, while the metal of telescopes and plumbing appear to be chrome plated. The scale and vernier are engraved on nickel-silver, while the crank/knob is machined aluminum. The scale is numbered from 0-72 over 7 5/8", with graduations to the tenths place, and a vernier reading to the hundreds place. According to the literature description of this instrument the scale is divided to an accuracy of 2 seconds of arc (approximately 1 micron in linear measure n the scale).3

1 From 1964-1985 B&L used a letter system for serial numbers, with a date letter added as a suffix, while the prefix letter is a class designation. Personal communication, Stuart Warter, California State University, Biology Dept., Long Beach State University (1998). The date letters are tabulated in Bracegirdle, Brian. Notes on Modern Microscope Manufacturers. Quekett Microscopical Club, Oxford (1996).

2 Straat, H. W. and J. W. Forrest. (1939) The Accuracy Requirements in Fifth Place Refractometry. J. Opt. Soc. Amer. 29: 240-247.

3 Forrest, J. W. (1938) Precision Refractometer Design. Proc. Intern. Soc. Sugar-Cane Tech., 6th Cong. (Louisiana) 890-5.

4 Bauer, N., K Fagans, and S. Z. Lewin (1960) Refractometry. in Technique of Organic Chemistry, Volume I Physical Methods of Organic Chemistry. Part Two. 3rd Ed. (Arnold Weissberger, ed.) Interscience Publisher, New York.

Refractometer Exhibit Catalog

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HSTC (1921-34)
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Last modified 23 August 2010