The common method of measuring optic angles from interference figures is that employing the Amici-Bertrand lens or one of its many modifications. The principle involved in the use of the Amici-Bertrand lens is to produce a diminished image of the interference figure which is subsequently magnified by the ocular. The resulting interference figure loses some of its distinctness and sharpness of outline because of this procedure. The diminished image of the interference figure which is produced by the Amici-Bertrand lens falls on the cross-hairs of the ocular. In most microscopes this reduction in size is of the order of one half to one-third. The ocular then magnifies the image according to the power of the ocular giving a resultant interference figure which is from two to five times the size of the interference figure that forms in the rear focal plane of the objective. In order to measure the optic angle a scale is substituted for the cross-hairs of the ocular. This scale is likewise magnified by the ocular in exactly the same ratio as the image of the interference figure and appears superimposed upon that image as seen by the observer. This method is commonly referred to as the Mallard1 method and the constant of the calibration curve as the Mallard constant.

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