-On some Properties of Light. 289 
bonate of strontites, jargon, and other crystals may give 
double images, in virtue of similar binary combinations« 
Of the simple inflammable bodies, sulphur is the only one 
which has the property of double refraction ; but it will pro- 
bably be’ found that it holds a metal or some other ingre- 
dient in its composition, which chemists have not been 
able to discover. ; 
_ If the explanation which has now been given of the po- 
larising power of the agate should be confirmed by future 
experiments, this property will be considered as a case, 
though a very curious one, of double refraction; but if 
these conjectures should be overturned, the phenomena 
which we have described must be ranked among the most 
singular appearances in the wide range of optical science. 
2. On the double Refraction of Chromate of Lead. 
In the course of my experiments on refractive powers, L 
discovered a double refraction in this metallic salt of such 
enormous magnitude, that the deviation of the extraordinary 
ray is more than thrice as great as that produced by Iceland 
spar. The ratio of the sines, for both refractions, and the 
other properties of this extraordinary mineral, will be noticed 
m the next article. 
3. On Substances with a higher refractive Power than the 
hart Diamond. 
~ Since the time of Sir Isaac Newton, who first measured 
the action of the diamond upon light, its refractive power 
has been regarded as superior to that of every other snb- 
stance; but, in the course of my researches, J have found 
that realgar and chromate of lead exceed the diamond in 
refractive power, and that this high refraction, in both 
these substances, is accompanied with dispersive powers 
gresler than those of any other body. The following are 
the measures which I have obtained for these, and a few 
other substances. 
Refractive Powers. 
Index of Refr. Index of Refr. 
Chromate of .lead Phosphorus ...... 2°234 
* (gr. refr.) ....... 2926 Sulphur, native..., 2115 
Ditto, least refraction 9°479 Cryolite......... 1°344 
WORT ac goin ys «ROTO LCE. cee eeccs eves LUE 
Diamond (according 
to Newton) .J... 2439 
Vol. 42. No. 186. Oct. 1813. TT Di- 
