Dr. Brewster on the laws of polarisation, &c. 207 
ence of two separate images is the only correct and infallible 
test of double refraction, I have employed it for the greater 
part of the substances in the table. When a crystal possesses 
the property of depolarising light, it only proves that it forms 
two pencils polarised in opposite planes ; though there can be 
little doubt that one of them 
is subject to the extraordinary 
law of refraction. 
Table of doubly ; 
refracting crystals. 
Chromate of lead 
Tourmaline 
Carbonate of lead 
Rubellite 
Tungstate of lime 
25 Quartz 
Carbonate of potash 
Agate 
5 Nitrate of potash 
Dichroite 
Calcareous spar 
Bitter spar 
Arragonite 
Apatite 
Zircon 
30 Idocrase 
Sulphur 
Mica 
10 Acetate of lead 
Lepidolite 
Acetate of nickel 
Talc 
Ruby 
Indurated talc 
Sapphire 
35 Chlorite 
Corundum 
Kyanite 
15 Beryl 
Stilbite 
Emerald 
Cubizite 
Cymophane 
Apophyllite 
Peridot 
4<o Prehnite 
Epidote 
Hydrate of magnesia 
20 Euclase 
— barytes 
Topaz 
— — — strontites 
Pycnite 
Feldspar 
