In fig. 30 the analyzer is represented as composed of a natural crystal (rhomb) of calcareous spar, which transmits both sets of rays; but in the ordinary analyzer or Nicol’s prism--which is made by dividing a rhomb through the obtuse angles into two wedge-shaped pieces and cementing them together again with balsam, only one set of rays is transmitted at each quarter-revolution, the other being refracted out of the field. In the case of the tourmaline, one of the sets of rays is absorbed; so that the tourmaline, like the Nicol’s prism, is single-imaged. Thus the colours produced by polarization are the same as those of the spectrum, but separated in a different way, both arising from the elementary coloured rays of the compound white light. For while the spectral rays are separated by dispersive refraction, the polarized coloured rays are separated by the interference and annihilation of some rays, the remainder passing on to produce the colours. When the position of the depolarizing crystal is such that the plane of the polarized light coincides with the direction of the optic axis or axes, the light is not doubly refracted nor polarized in certain parts; hence these parts appear white if the plane of the polarizer and analyzer coincide, and black if they be crossed. A crystal cut at right angles to its optic axis, with its length directed towards the polarizer and analyzer, is in this position, and it exhibits alternately a black and white cross, with one or two sets of concentric rings of complementary colours at each quarter of a rotation. To prepare crystals for examination by polarized light, a little Epsom salt, nitre, or borax should be dissolved in water, a drop placed upon a slide, and dried at a gentle heat. The crystals should then be mounted in balsam, and viewed as transparent objects. To see the cross and rings, the crystals should be sawn or cut across transversely, the ends being polished on a strained piece of silk moistened with water, and the sections mounted in balsam. The property of doubly refracting and polarizing light is not confined to crystalline substances, being also possessed by many organic bodies, for the details of which I must refer to the article Polarization in the ‘Micrographic Dictionary.’ INDEX. Abdomen, 129. Aberration, chromatic, 173. Aberration, spherical, 172. Acalephæ, 153. Acarus, 131. Accumbent, 44. Achromatic, 178. Achromatic condenser, 5. Achromatism, 177. Acicular, 24. Acotyledons, 43. Acrocarpi, 55. Acrostalagmus, 111. Actinocyclus, 80. Actinophrys, 156. Adapter, 2. Adulteration, 23. Æcidium, 100. Agaricus, 97. Albumen, of seeds, 43. ----, of eggs, 113. Algæ, 64. Alyscum, 161. Amœba, 155. Amphileptus, 162. Angiocarpi, 94. Angle of vision, 175. Anguillula, 152. Angular, 23. Animal tissues, 113. Ankistrodesmus, 74. Annular vessels, 27. Annulus, of Ferns, 50. ----, of Fungi, 98. ----, of Mosses, 55. Anoplura, 135. Ant, 143.