262 
Proceedings of the Boy at Physical Society. 
one belonging to the prismatic system. Biaxal crystals 
belonging to the oblique and anorthic systems may, for the 
purposes of this investigation, be considered as formed by 
the combination of hemihedral and tetrahedral forms of the 
prismatic system. 
In the prismatic system the molecules are symmetrically 
arranged with respect to three planes at right angles to each 
other. Also, if a very small crystal be taken out of a larger 
crystal by cleavage or otherwise, it is also biaxal, and possesses 
optical properties similar to those of the larger crystal, the 
optic axes being in the same direction, &c. The larger 
crystal may therefore be considered to be built up of aii 
indefinite number of very small crystals of the prismatic 
system, and each of these small crystals may be supposed to 
contain all the material molecules which are capable of 
influenciug the motion of the ether within them. Hence at 
every point of a crystal we may suppose that the molecules 
which can affect the motion of the ether at that point are 
symmetrically arranged with respect to three planes at right 
angles to each other, and parallel to the principal planes of 
the crystal. 
Hence 
2 I A fr h' fc' 1 = 0, 
&c. 
...^ = 0 <J^=0,by(18)and(18*). 
And ^ = - «'2 I i /V K = + fr } = - P, say 
.-, '^^^-u^^-^^ V + ^w= - Vu, . . . (19) 
ax dy dz 
Also hir^ — i) — -f u + -r4 + 7 / \ 
\ dx dy dz ) 
Hence by analogy, from the value of 5^.^, we may put 
a..=P.'+A'5\B'|!;'.0'g^. . (20) 
where A' B' C are constants depending on the arrange- 
ment of the material molecules, that is, on the crystalline 
