Page:Theophrastus - History of Stones - Hill (1774).djvu/309

 that Body from the pure Rhombic Spar; which they uppofed not to have the double Refraction. But this Power reides in all Spar I have examined: And is of its Nature: As it aries from the internal Contruction of the Body, which is made up of maller Rhombs, applied one to another.

The very Atoms of Spar are Rhombic; and thoe mallet Pieces into which it may be eparated by gentle Acids, without Solution, applied to the Microcope over a Line proportionably fine, have the ame Power.

No Body has this Contruction except Spar; therefore no other natural or artificial Subtance has this Power of double Refraction. Even Sir Iaac Newton has aid, Crytal has omething of this Power; in vain: For no Authority can tand againt the Tetimony of the Senes. All different Mediums vary in Refraction; but this peculiar Power reides only in a pellucid Body formed of connected Rhombs.

The State of Refraction in the pellucid natural Bodies is this,

All Spar does this, even that which takes the Form of Crytal, in Pyramids, and Columns: Therefore even the variouly angulated Forms