'Natural Pkilosbph^^ — Vpfics. 3T9 
OPTICS. 
% Mr HerscheVs Experiments on Plagiedrccl -In our 
4th vol. p. 371. we have given a short abstract of Mr Herscliel’s 
ingenious paper on Circular Polarisation, in which he announced 
the very important fact, that the direction of the circular polari- 
sation coincided with that of the plagiedral planes, in no fewer 
than twenty-three crystals, without a single exception. Mr 
Herschel informs us that he has more recently examined thirty 
additional crystals of quartz from Mont Blanc, all of which ex- 
hibit the same relation ; so that the generality of the fact may 
now be considered as established beyond a doubt. Mr Herschel 
has likewise observed the curious fact, that the number of pla- 
giedral crystals in which the planes have an inclfnation to the 
left, are to those which have them in the opposite direction near- 
ly as 2 to 1. 
3. New Pf'operiy of the Ordinary Ray of Crystals with two 
Axes. — M. Fresnel has recently discovered, that in crystals with 
two axes of double refraction, the ordinary ray undergoes va- 
riations of velocity and refraction, analogous to the extraordi- 
nary ray, but confined within less extended limits. — Bihl. Uni- 
verselle^ Dec. 1821, p. 267. 
4. Smgular Effect of Heat on the Colouring Matter of Ruby. 
-—In subjecting rubies to high degrees of heat, Dr Brewster ob- 
served.a very singular effect produced during their cooling. At 
a high temperature, the Red Ruby becomes Green ; as the cool- 
ing advances, this green tint gradually fades, ^nd becomes 
Brown,, and the redness of this brown tint gradually increases 
till the mineral has recovered its primitive brilliant red colour. 
A green ruby suffered no change from heat ; and a bluishrgreen 
sapphire became much paler , at a high heat, but resumed its ori- 
ginal colour by cooling. 
ELECTRICITY. 
5. Singular Effect cf Lightning at Professor Pictet 
communicated to the Helvetic Society an account of a singular 
effect produced by a stroke of lightning on the 3d of July last* 
The house had no conductors, but its roof was covered with 
white iron, and had bars off the same metal communicating 
witli the ground. The stroke of lightning did no damage 
