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Updated: May 26, 2025
Here, as elsewhere, the different lengths of the light-waves give rise to iris-colours when white light is employed. Besides the regular crystals which produce double refraction in no direction, and the uniaxal crystals which produce it in all directions but one, Brewster discovered that in a large class of crystals there are two directions in which double refraction does not take place.
For instance: if we tell them that in 1820 he wrote a paper "On the Theory and Summation of Series;" communicated to the Cambridge Philosophical Society his discovery that the two kinds of rotatory polarization in rock crystal were related to the plagihedral faces of that mineral; and issued an able treatise "On Certain Remarkable Instances of Deviation from Newton's Tints in the Polarized Tints of Uniaxal Crystals," they will gain no very distinct idea of the significance or value of these researches.
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