Humboldt State University
Robert A. Paselk Scientific Instrument Museum
Brice-Phoenix Differential Refractometer
Phoenix Precision Instruments
Chemical Heritage Foundation
According to the designers of this instrument, "The measurement of small differences in the refractive index of liquids has found increasing use in recent years [late 1940's], particularly in the determination of high molecular weights by the method of Debye [light scattering], in following fractionation by distillation or absorption, indetermining concentrations of colorless solutes in dilute solutions, and in studies of sedimentaion equilibrium."2 These determinations generally required an accuracy of a few units in the sixth decimal place of refractive index, an order of magnitude better than the fifth-place accuracy expected from the dipping, Pulfrich, or high precision Abbe refractometers in common use. The Brice-Phoenix (absolute deviation) differential refractometer has a sensitivity of about 3 x 10-6 units of refractive index with an accuracy of 0.5%. This makes it particularly useful for determination of refractive index increments needed in determining high molecular weights by light scattering, and small differences in concentration.
A brief essay, The Chemical Refractometer, describes the characteristics, design, and use of various refractometers.
The instrument is built on a four foot long (note the meter stick in the photo for comparison), heavy cast aluminum, lathe-bed style optical bench. The instrument is 48 1/2" long in its greatest dimension, 13 1/2" to the top of the lamp housing, and 8" wide at its widest point (across the leveling foot tabs on the casting. Beginning with the right side of the instrument in the photo there is a lamp housing (8"h x 4 1/2" x 4 1/2") enclosing an arc lamp with a standard (household) screw-base. There is a four position machined aluminum filter wheel on the front of the housing, with a 1" blue filter in one position, the others being empty. The next position on the bench is occupied by an empty 2" filter holder clamped at a 45° angle to the optical path, originally used for a beam splitter. In the next position is an adjustable slit stamped Phoenix Precision Instrument Company. This is followed by the water jacketed cell holder, with a lever allowing precise (adjustable) 180° rotation of the cell. (The cell holder cover is missing.) The 1.5 cm pathlength cell is split with an angled window into two compartments to allow comparison of known and unknown samples. Next in line is a plastic circular bubble level, followed by a "Wollensak 5 3/8" (138 mm) f 4.5 Graphic Raptar" lens with diapraghm closing to f 32 in a 2" screw mount. Finally, an AO Spencer instrument microscope ( ser. no. 565047) with a 3x NA 0.08 objective is mounted as a viewing telescope. A Bausch & Lomb filar micrometer is used as the eyepiece/readout. With the exception of the lamp housing, which is attached directly to the optical bench, all components are held by optical mount carriers which clamp to the bench, and all but the level are on half-inch rod mounts. The actuall spacing of the components is, however restricted by the stainless steel cover, which has cut holes at specific distances. Decorative brushed-aluminum sleeves coever the tops of the carriers above the cover, with black plastic washers between the cover and the sleeves.
1 The microscope was dated by its serial number to 1965 using data provided by Michael Benz of Benz Microscope Optics Center (personal communication, c. 1998). A less detailed listing is available in Bracegirdle, Brian. Notes on Modern Microscope Manufacturers. Quekett Microscopical Club, Oxford (1996).
2 Brice, B. A. and M. Halwer (1951) A Differential Refractometer. J. Optical Soc. Am. 41, 1033-7.