243 
of Edinburgh^ Session 1863 - 64 . 
Id the aDorthic system, the position of the axes of elasticity is 
subject to no known law. 
3. These facts show that there is a fundamental connection be- 
tween crystalline form and optical properties. Of late, nevertheless, 
many eminent physicists, following the views of De Senarmont, have 
doubted the existence of such a connection. We shall, therefore, 
show hereafter that the experiments of De Senarmont as well as 
those of Des Cloizeaux are not at all opposed to, but important 
confirmations of, the views here adopted, which are founded on the 
facts stated above, and in addition on the remarkable relations that 
have been observed between hemihedral forms and rotatory polarisa- 
tion in quartz, and dextro- and Imvo- tartaric acids. 
4. In this note we shall consider in detail the connection between 
the form and optical properties of crystals belonging to the pris- 
matic system. For crystals belonging to the oblique and anorthic 
systems, the investigation is still in progress. 
5. We proceed to consider the following problem. 
“ Griven the angular elements of a crystal belonging to the pris- 
matic system, to find the angle between the optic axes.” 
We have before observed, that, in the prismatic system, the 
optic axes always lie in a plane containing two of the crystallo- 
graphic axes, and make equal angles with each of these axes. 
First, let the optic axes lie in the plane containing the para- 
meters c and a, and each make an angle with the axis c. Our 
object is to find an expression for some function of w,, say tan 
in terms of the parameters a, b, c. 
Now crystals belonging to the pyramidal system have only one 
optic axis, which coincides with the axis c, if a — h. Hence, when 
a = b, the angle which each optic axis makes with the axis c 
vanishes ; or, in other words, both optic axes coincide with the 
axis c. 
Hence, co^ and tan vanishes when a = b. 
tan is divisible hy a - b. 
6. In an abstract of a paper on the “ Khombohedral System, ” 
published in the present number of the Proceedings,” it is shown 
that the crystals hitherto included in the rbombohedral system are 
merely the particular forms which crystals of the prismatic system 
assume when one angular element is equal to 60°. 
