Another marvelous property of crystals and crystalline substances is their effect upon light. Since the study of the passage of light through crystals has really become a large separate branch of mineralogical study, we can no more than state a few fundamental facts and principles in the short space at our disposal. Light is caused by vibrations of the so-called "ether," and always travels in straight lines. The vibration directions are at right angles to the direction of transmission of the light. When a ray of light enters a crystal or crystalline mineral representing any crystal system except the isometric it is doubly refracted (i.e., broken into two rays), each of the two rays is polarized (i.e., made to vibrate in a single plane only), and one ray vibrates almost at right angles to the other. Double refraction is strikingly shown by placing a piece of clear calcite (Iceland spar) over a dot on paper when two dots instead of one are visible. The amount of double refraction varies with the substance, and in some degree according to the direction of passage of light through a crystal. Isometric crystals only are singly refracting and hence a ray of light is not affected in passing through them. Crystals of all the other six systems doubly refract and polarize light and in three systems--tetragonal, hexagonal, and trigonal--one direction (coincident with the main axis of symmetry) produces single refraction only, while in the remaining three systems--orthorhombic, monoclinic, and triclinic--there are always two directions of single refraction whose positions vary with the substance. Many crystals outside the isometric system also exhibit a remarkable tendency to absorb light differently in different crystallographic directions, thus producing two or three color tints, which vary according to the substance. After gaining a practical knowledge of the above and many other optical properties of crystals, it is possible by the aid of a specially constructed (polarizing) microscope, to recognize (with few exceptions) each one of the many mineral species. This method is of great value in determining the various minerals which are aggregated in the form of a rock, in which case a very thin slice of the rock is studied with the microscope. An important criterion for the recognition of minerals is hardness, by which is meant the resistance of a smooth surface to abrasion or scratching. The generally adopted scale of hardness follows: 1.--Soft, greasy feel, and easily scratched by the finger nail (e.g., talc). 2.--Just scratched by the finger nail (e.g., gypsum). 3.--Just scratched by a copper coin (e.g., calcite). 4.--Easily cut by a knife, but does not cut glass (e.g., fluorite). 5.--Just scratches soft glass, and is cut by a knife (e.g., apatite). 6.--Harder than steel, and scratches glass easily (e.g., orthoclase).