VIII. On the Optical Properties of Muriate of Soda. Fluate of 
Lime, and the Diamond, as exhibited in their action 
upon Polarised Light. By Davw Brewster, LL. D.. 
F.R.S. Lonp. & Epiw. & F. A. S. Enin,, 
[Read Nov. 20. 1815.] 
WT has long ago been remarked by the Abbé Havy, that’ the 
i property of Double Refraction is not possessed by any of those 
crystals the form of whose integrant molecule is distinguished 
by its symmetry, such as the Cube and the Regular Tetraedron. 
_ This class of minerals comprehends Fluate of Lime, Muriate of 
Soda, Spinelle Ruby, Muriate of Ammonia, Alum, and the Dia- 
mond; and though M. Havy had examined only a very small 
number of doubly refracting crystals, yet, with the exception 
of the Diamond, which I have found in many cases to possess 
the property of Double Refraction, his remark was confirmed 
by the experiments of Matus, Bror and myself, who consider- 
ed all the crystals of this class as exercising no more action 
upon polarised light than a mass of water. 
No explanation was offered of this singular anomaly, till 
M. Bior discovered that all doubly refracting crystals are 
divisible 
