Humboldt State University
Robert A. Paselk Scientific Instrument Museum
Fisher Scientific Co.
The Fisher refractometer is based on a microrefractometer described by E. E. Jelly in 1934. 2 The optical principle is shown in the catalog description in the scan above. Essentially a small prism of the sample is created by placing it into the wedge shaped hole created between the prism and the window on the instrument. The Fisher refractometer can be read directly to 0.002 units of refractive index over a range of 1.30-1.90. The original Jelly design used a microscope cover glass and only required 0.0001 mL of sample. The more robust prism of the Fisher instrument can be used with as little as one microliter. The Fisher refractometer is very convenient and relatively inexpensive, but can only measure n accurately to about 0.002 rather than the 0.0002 of an Abbe refractometer. The instrument in this exhibit is equipped with a heater, allowing the observation of refractive index in specimens with melting points up to 200°C. It can also be used to estimate purity of specimens by observing changes in refractive index as the specimen evaporates. The catalog scan is from: Fisher Scientific Company Modern Laboratory Appliances, Catalog 59. New York (1958).
The instrument stands 11 3/4" high to the top of the eye-shield as seen in the photo. The base is 5" w x 9 3/4" d. The instrument is made from a single aluminum casting, finished in gray crinkle-enamel. The glass prism is 0.281" x 0.300" x 0.047" with a 45° angle along the 0.281" edge. It is stored under the aluminum Fisher tag on the base of the instrument (see photo. The original typed instructions are also in the collection.
1 The 1949 Fisher catalog describes a crinkle finish as seen on this instrument, while the 1959 Fisher catalog (see catalog scan) describes the instrument as now having a "smooth hammertone finish."
2 Jelly, E. E. (1934) J. Roy. Microscop. Soc., 54, 234. Referenced in Bauer, N., K. Fajans, and S. Z. Lewin. "Refractometry." in Physical Methods of Organic Chemistry Vol. I, Pt II, 3rd ed.; Arnold Weissberger, editor, Interscience publishers, Inc., New York. (1960) p1276.