The important features of two achromatic slit images, which appear near minimum deviation for an isosceles prism, are described and applied for refractive index determinations. Either of these images can be utilized in a prism orientation technique equally suited for visible and non-visible wavelengths. This technique has been tested by determining the refractive index curves (0.4 to 2μ) of isosceles prisms prepared from six Eastman Kodak glasses. Another tested technique is presented whereby the angular separation of these two images is used to determine the angles of an equilateral prism with a negligible uncertainty.
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