ELONGATION relates to the appreciable extension often shown by the crystal section. A crystal of long prismatic habit, cut about parallel to the _ć_ axis, would show marked elongation; while a tabular crystal (like mica) would show elongation if cut at right angles to the tabular faces. At times, of course, no elongation is appreciable, as in the case of granular or broken crystals or where the cross-section is essentially square or octagonal. Very often the relation of the cleavage is given to the elongation and also to the directions a′ and c′, which makes it possible to test for a′ and c′ even when no marked elongation can be observed. MINERALS IN ROCK SECTIONS. CHAPTER I. INTRODUCTORY OPTICS FOR OPTICAL MINERALOGY. The object of this introduction is merely to give a practical discussion of elementary optics, as applied to optical mineralogy,[3] and no elaborate discussion of this important subject will be attempted. The explanations will be made as simple as possible, and, in most cases, only the optical phenomena will be described without entering into a theoretical discussion as to the cause of these phenomena. _Light_[4] may be regarded as transmitted in straight lines by vibrations of the “ether,” taking place at right angles to the direction of transmission. _Ordinary light_ is light with the ether vibrations in all possible directions, the path described by any particle of ether constantly changing. _Plane polarized light_ is simply light with the ether vibrations all _parallel_ to one plane passing through the direction of transmission. By experiment it has been proved that there exists a very close relation between the optical properties of crystals and their other physical properties, such as form, color, transmission of heat, etc. Therefore it is often possible, by a careful optical investigation of a crystal section, to determine important crystallographic facts, even in the absence of any distinct outline. The Effects Produced by Crystals on Transmitted Light. Consider that a series of optical tests are made on all possible sections[5] of crystals in the six systems, and the manner in which these crystals affect transmitted light ascertained. ▄Isotropic Crystals▄: It will be found that all sections of _Isometric_ crystals transmit light with equal velocity in all directions; that is, the crystals are optically equivalent in all directions and, hence, can produce no double refraction.[6] In these crystals any section, however cut, will transmit all the rays of light, incident to the surface at right angles, with no change.[7] The same is true of _Amorphous_ bodies, glass, etc., unless they have been subjected to strains or peculiar conditions during cooling. A _single image_ is seen through these isotropic sections. [Illustration: FIG. 1. ]