335 
quantities we find by making use of some simplifications that are 
allowed because of the smallness of this difference: 
n° d° w(u?— 1)? 
2 
(1) 
LY 
uy py, =; 11 
ne 
Here n is the number of vibrations in the time 2ar, d the distance 
of the molecules viz. the edge of the cubic lattice and w one of the 
refraction indices u, and uw, or rather the mean of both; we may 
say the observed refraction index. | 
From (1) we see that the sign of u,—u, is the same as for a 
plate of calcite the optical axis of which coincides with the direction 
R, and also the same as for a glass plate that is compressed in the 
direction &,. 
During the propagation in the crystal over a distance D a diffe- 
rence of phase arises between the vibrations in the directions Rk, 
and R&,. Expressed in periods or wavelengths this difference in 
phase is determined by 
Do? 
Ae 
a 
wo = (u,—t,) rice 0,44 wu (u?— 1)? RR HP (7) 
where 2 is the wavelength in vacuum. 
For rock-salt d = 2,80 .10-8 cm. 
With this d we find for a thickness of 1 em the following 
values of w. 
fo enorm heeht); 5,1 74103: 107% BS 210 
wm = 0,016 - 0,025 ; 0,14 EE bs 
Even in the visible spectrum these numbers are great enough to 
let us expect that under favourable circumstances the effect of the 
double refraction will be detectable between crossed Nicols. 
4. I have sought for the phenomenon in several pieces of rock- 
salt, in which the faces of the cube were obvious and which were 
bounded by two side-faces perpendicular to the direction indicated 
above with Z; the distance between these phases was about one cm 
The side-faces could be easily polished, but we met with the diffi- 
culty, that they loose their polish even when the crystal is kept in 
dry air. To avoid this the crystal was put into a tube a little longer 
than the tickness of the rock-salt and shut on both sides by glass 
plates from Hiterr with neglectable double refraction. The remaining 
space in the tube was filled with a mixture of carbonic disulphide 
and benzol, of which for the mean yellow light, the refraction index 
is equal to that of the rock-salt. Under these circumstances the 
erystal is beautifully transparent even when the side-faces have not 
