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that Body from the pure Rhombic Spar; which they ſuppofed not to have the double Refraction. But this Power reſides in all Spar I have examined: And is of its Nature: As it ariſes from the internal Conſtruction of the Body, which is made up of ſmaller Rhombs, applied one to another.
The very Atoms of Spar are Rhombic; and thoſe ſmalleſt Pieces into which it may be ſeparated by gentle Acids, without Solution, applied to the Microſcope over a Line proportionably fine, have the ſame Power.
No Body has this Conſtruction except Spar; therefore no other natural or artificial Subſtance has this Power of double Refraction. Even Sir Iſaac Newton has ſaid, Cryſtal has ſomething of this Power; in vain: For no Authority can ſtand againſt the Teſtimony of the Senſes. All different Mediums vary in Refraction; but this peculiar Power reſides only in a pellucid Body formed of connected Rhombs.
The State of Refraction in the pellucid natural Bodies is this,
1. Talc in thick Maſſes elevates the Line.
2. Selenite waves it.
3. Cryſtal diſtorts it,
4. Spar gives it double.
All Spar does this, even that which takes the Form of Cryſtal, in Pyramids, and Columns: Therefore even the variouſly angulated
