160 Scientific Proceedings, Royal Dublin Society. 
is repeated, but the triradiate group is turned round through 60°, 
so that the -N-O-Na rays below lie midway between those 
above, and thus correspond to the three lower edges of the 
rhombohedron. Finally, the upper and lower moieties are united 
by six atoms of oxygen, which are linked to the nitrogen atoms, 
two of oxygen to each atom of nitrogen. Thus, each crystal 
molecule consists of six chemical molecules, each of the composi- 
tion NO;Na. It may be regarded as distributed in two differently 
directed groups, one consisting of oxygen (and so muck of the 
nitrogen as belongs to it), six atoms of which are vertically 
linked, 7.e. parallel to the optic axis, to a nitrogen atom; and the 
other of six rays of -O-N-O-Na, the chemical bonds of which lie 
more or less parallel to the equatorial plane. It is essential to 
observe, that of the nitrogen atoms, only three out of five bonds 
of each are horizontally linked, and consequently we are led to 
assign only three-fifths of the molecular weight of this element to 
the equatorial members of the molecule, leaving the remaining 
two-fifths to the oxygen, which is linked vertically. 
Let us now suppose a ray of light to enter our molecule along 
the optic axis, its wave front will lie in the equatorial plane or 
that in which the Na-O-N-O- members are supposed to act, and 
since in the direction of the optic axis there is only ordinary 
refraction we may connect these members with the ordinary 
refractive index. If, on the other hand, the path of the ray be 
parallel to the equatorial plane it will be resolved into an ordinary 
ray, the transversal of which being in the equatorial plane will be 
related to the equatorial members of the molecule, and an extra- 
ordinary ray, the transversal of which, being vertical, will be 
related to the vertically linked components. In this direction, 
in an optically negative crystal, such as sodium nitrate, the 
refractive index of the extraordinary ray has its minimum 
value, and it is this value which stands in direct relation to 
the refractive energy or refractive equivalents of the vertically 
linked atoms. . 
We are now in a position to apply our optical probe. If the 
structure of the crystal be that which we have suggested, the 
lower refractive index should be that due to the refractive energy 
of the vertical components of the molecule, and the higher, that 
due to the refractive energy of the equatorial component. 
