The Polarizing Microscope.— The various polariscopes of former days have been replaced almost entirely for crystal study by the polariz ing microscope the simplest form of which necessarily includes the essentials of an ordi nary microscope (a stand supporting a tube carrying objective and ocular, a platform or stage and means of focusing and illuminating), and in addition a nicol, «the polarizer,)) below the object, to polarize the incident light, and another above the objective, «the analyzer» ar ranged to be thrown in or out and °crossecP with respect to the polarizes.
To these essentials may be added many de vices and attachments for special purposes.
The tests usually are made in known crystal lographic directions, preferably upon «plane parallel)) sections or through natural faces at right angles to the desired direction of trans mission.
«Crossed the vibration di rections of the upper and lower nicol are crossed, the polarized light from the lower nicol cannot penetrate the upper nicol and the field is dark. If when a transparent section of a crystal is placed on the stage and revolved, the field is still dark, the direction of transmis sion is one of single refraction, hut if during the rotation the field is in general illuminated, but is dark for four positions, 90 degrees apart, then the direction of transmission is one of double refraction.
These four positions of darkness or tioe occur when the vibration directions of the two polarized rays emerging from the section are parallel to those of the two nicols for then only the polarized light from the lower nicol is transmitted by the section with unchanged vibration direction and cannot pass through the upper nicol.
With crossed nicols and a doubly refracting crystal section the light effect at any point is due to the combined effect of two components with vibrations in the same plane. In general they have been unequally retarded during trans mission and may supplement or neutralize each other. If for instance monochromatic light is used and the difference in retardation of the two rays is denoted by A and the wave-length of light by X then: If A'=' 71, 271, 371, .. , n 71 the two compo nents following the same path will oppose and exactly neutralize each other, the light will be stopped and darkness will result for all posi tions of the crystal section.
Similarly if z ----- Y2A, etc, the maximum illumination results.
Since A increases with the thickness these facts may be shown by use of a thin wedge of a double refracting crystal. Under low magnifi cation this will show with monochromatic light equidistant dark bands with brightest illumi nation midway between them. When white light is used instead of monochromatic light ((interference colors° result because for any section z may be at the same time: (a) ap proximately a multiple of the wave-length of light of one or more colors, which would, there fore, be shut out: (b) closely an odd multiple of the half wave-lengths of other colors leaving these at nearly their full intensity, and (c) in termediate, with still other colors.
For any value of A there is an absolutely fixed interference color and as these to some extent repeat periodically they are conveniently divided into uorder0 the transition color be tween orders being a sensitive violet so-called be cause very minute changes in i result in de cided changes to blue or red. The first, second and third orders end respectively at A = 575, 1,130, 1,652 millionths of a millimeter.
Determining the Fact of Single or Double Refraction.— If with crossed nicols and the crystal section on the stage the field remains dark throughout the rotation of the stage the crystal is singly refracting in the direction of transmission. If the field is dark at intervals of 90 degrees and elsewhere illuminated the crystal is doubly refracting.
Determining Indices of Refraction:—As curate determinations require monochromatic light, constant temperature and, except in iso tropic substances, those directions of transmis sion which yield the principal indices. The more used microscopic methods are those of Becke and Van der Kolk both of which involve a com parison with the known indices of liquids.
Becke's method is based upon total reflection at an approximately vertical boundary between two substances and the consequent concentra tion of light upon the side of the substance with the higher index of refraction. After focussing sharply upon a fragment or grain with an ap proximately vertical boundary or edge and sur rounded by the liquid, the objective is slightly raised, giving the effect of a bright band moving into the denser substance.