Humboldt State University
Robert A. Paselk Scientific Instrument Museum
The Pulfrich refractometer was advertized as being particularly suited for 1) measurement of refractive index and dispersion for any given light (e.g. sodium D lines, Hydrogen, C, F, G' etc.) of all transparent liquids and solids, 2) studies of fluids at high temperatures, and 3) measurements of differences of refraction or dispersion over small ranges. The basic theory of the instrument is diagramed and explained in the catalog scan above. The Pulfrich refractometer was considered a higher precision and more flexible instrument than the standard Abbe refractometer. Though its direct reading specifications show it to be good to 0.0001 unit of refractive index (one part in the "fourth place") like the Abbe, as a differential instrument it is good to 0.00002 (two parts in the "fifth place"). Thus the graduated circle can be read to one minute by vernier for direct measurements, while the tangent screw, which clamps to the graduated disk, has a range of six degrees, and is divided to 6 seconds for differential measurements.2An improvement of the Hilger instrument over its Zeiss predecessor is better temperature control via the jacketed prism cover.
The original "Refractometer for Chemists" was designed by Carl Pulfrich and described in 1888.3 This earlier, somewhat simpler instrument was constructed and sold by Max Wolz in Bonn, Germany. The Hilger instrument in this exhibit is based on the instrument Pulfrich designed after joining Carl Zeiss, and was described by him in 1895.4 This second version of the refractometer for chemists was sold as the Pulfrich Refractometer by Carl Zeiss, and has been known as a Pulfrich refractometer since. Adam Hilger introduced their version of the Pulfrich refractometer in c. 1916.5 A comparison of the engravings for the Adam Hilger instrument in the exhibit (Plate I from the 1916 manual) and the Carl Zeiss original (Fig 1 from the 1895 Pulfrich manual) shows the closeness of the two designs. The catalog scan is taken from Arthur H. Thomas Co. Laboratory Apparatus and Reagents. Philadelphia (1921).
A brief essay, The Chemical Refractometer, describes the characteristics, designs, and use of refractometers.
The instrument stands 15 7/8" high in the closed position without a thermometer. The inlaid 'silver' divided circle is 5" in outer diameter, set in a 5 1/4" diameter brass circle with knurled edge.The tripod base is of heavy cast iron, with a japanned finish. The rest of the instrument is brass,, except for the telescope, which is of aluminum, and lower heat exchange finger/thermometer holder which is copper. All exposed metal parts appear to have been polished by a previous owner.
The instrument has its original hand-dovetailed mahagony case (17 1/2"h x141/4"w x 12 1/5"d; the latch-hooks and leather handle appear to be replacements), two prisms (nD = 1.62 and nD = 1.74), and its original instruction manual with appendix addendum.
1 According to Peter Morris (Science Museum, London) the serial number before the slash gives the model number (M48) followed by the number of the example (302). The two digits immediately following the slash tell the year of manufacture (1920).
2A detailed treatment of refractometry and refractometers is given in: Tilton, Leroy W. and John K. Taylor. "Refractive Index Measurement." in Physical Methods in Chemical Analysis Vol. 1, 2nd ed. ; Walter G. Berl, editor (1961) pp41162.
3 Pulfrich, C. (1888) Zeitschrift für Instrumentenkunde. 8. p 47; C. Pulfrich (1890) das Totalreflectometer und das Refractometer für Chemiker etc. W. Engelmann, Leipzig.
4 Pulfrich, C. (1895) Zeitschrift für Instrumentenkunde. 15. p 389; C. Pulfrich (1895) Zeitschrift für physikalische Chemie. p 294.
5 The copyright date of the Adam Hilger Pulfrich manual is 1916. The Pulfrich refractometer was also not included in the 1914 Adam Hilger catalog.