256 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
The author concludes with a brief description of the method of 
manufacture and mode of testing these new glasses. The making of 
silicate glass takes close upon three weeks. The crucible is heated 
during four or five days until it attains a red heat ; the inside is then 
well glazed out with molten glass of the kind to be made. The mixture 
of substances of which the glass is to be made is then placed in the 
crucible, thoroughly melted and worked into a homogeneous mass. The 
glass is then tested, and if in good condition is taken out of the oven 
and allowed to cool down a little, when it is transferred to another oven 
where it is left about three days to cool. The crucible is then broken 
up, and the clear transparent pieces of glass are next subjected to the 
“ setting ” process, which consists in heating them in moulds to about the 
melting point, in a special oven, to which a cooling oven is attached. 
The cooling takes ten to twelve days. The usable glass amounts to 
about 20 per cent, of the quantity melted. For special glass a process 
of fine annealing is used in which the glass is allowed to cool very 
slowly in a vessel the temperature of which can be accurately measured. 
Plane of Polarization and Direction of Vibration of the Light in 
Doubly Refracting Crystals.*— Prof. V. v. Ebner examines the vexed 
question of the direction of vibration of plane polarized light. He 
states as the fundamental data of Fresnel’s theory the two following 
propositions : — 
(1) That in plane polarized light the oscillations take place at right 
angles to the direction of propagation in one plane, which is either the 
plane of polarization or one at right angles to it. 
(2) That the velocity of propagation of the polarized light in a 
crystal depends only on the direction in the crystal in which the vibra- 
tion takes place, and not on the direction of propagation. 
The first proposition is now undisputed ; but not so the second. 
We know, however, that longitudinal vibrations of the light- waves, at least 
in the interior of a crystal, do not come into account, for otherwise a 
rectangular crossing of the planes of polarization could not produce 
darkness. Further, along one and the same direction in a doubly re- 
fracting crystal the faster and slower waves can propagate themselves. 
Thus the direction of vibration is of essential importance for the 
velocity of propagation. 
Taking the case of an optically uniaxial doubly refracting crystal, 
the ordinary ray, according to the conventional expression, is polarized 
in the principal section, the extraordinary in the plane at right angles. 
In a sphere formed out of such a crystal every diameter will be a 
possible direction along which the light movement in the crystal can 
take place. One of these diameters must coincide with the optic axis, 
and planes through this diameter are principal sections. All these meri- 
dional planes are polarization planes of the ordinary ray, and tangents 
to the meridians cutting the surface of the sphere represent all possible 
directions of vibration which belong to one of the two polarized light- 
waves (it is yet undecided to which). 
Of the planes of the great circles which are at right angles to prin- 
cipal sections, the equatorial plane is distinguished as cutting all the 
* Zeitschr. f . wiss. Mikr., ix. (1893) pp. 289-97. 
