Refractometry of fluids in microscopic tubes.
Fluid filling a tube immersed in a medium that matches the index of the tube acts as a cylindrical lens. Because of geometrical aberrations, the focal length cannot be used directly for an accurate determination of refractive index. By observing the focal position with a microscope that has an aperture regulating diaphragm in the back focal plane, the ambiguity can be removed. The diaphragm restricts the observing light to two beams of equal and opposite obliquity. With unpolished tubes accuracy is about as good as with an Abbe refractometer, a few units in the fourth decimal place.
Saylor
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