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			$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	10. Transparent
				11. Translucent
	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Light.		12. Opalescent
				13. Chatoyant.
Substance.				14. Opaque
			$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	15. Iridescent
		Colour.		16. Monochroic.
				17. Pleochroic.
				18. Fluorescent.

The greater number of these terms will be found illustrated in the present and succeeding chapters: we now proceed to the discussion of the qualities which underlie them, and of other important physical characters of precious stones. The order which will be followed may be gathered from this scheme:

• Refraction.
• Dispersion.
• Polarization.
• Pleochroism.
• Hardness.
• Specific Gravity.
• Form.
• Structure.

Refraction of Light.—The familiar experiment of plunging a stick in a vessel of water and observing the broken appearance which it assumes, serves to illustrate the action called "refraction," or bending back. This refraction of light occurs in the majority of cases where a ray of light falls upon one transparent medium from another—say from the air upon a diamond. Part of the incident light enters the diamond, and follows a different path—is refracted. The diamond, like liquids, glass, and other molten or vitreous—that is non-crystalline—matter, possesses the property of simple refraction; many precious stones, indeed the