482 PROFESSOR MILLER, ON THE POSITION OF THE AXES OF 



Borax, Acetate of Soda, Felspar, Tartaric Acid and Gypsum, when heated 

 to about lOO^C, as was first observed by Mitscherlich, it is the greatest 

 or least axis of elasticity, and is therefore in the same plane with the 

 optic axes and makes equal angles with them. 



The position of one axis of elasticity having thus an evident rela- 

 tion to the crystallographic form, we are naturally led to inquire if 

 any relation can be discovered between the other two axes of elasti- 

 city and the crystallographic form. The only attempts to discover any 

 such relation, with which I am acquainted, are those of M. Soret, 

 (Memoires de la Socidte de Physique de Geneve, tome I.) and Pro- 

 fessor Neumann of Konigsberg (Poggendorff's Annalen, B. xxvii. S. 

 240). Neumann shews, that in Gypsum the axes of elasticity and also 

 the thermal axes, or the three lines in the crystal which remain at 

 right angles to each other at all temperatures, constitute a system of 

 rectangular crystallographic axes. It appeared at first sight not im- 

 probable that a similar relation might be found to exist between the 

 form and axes of elasticity of other oblique-prismatic crystals. Though 

 my observations appear to disprove the law which has thus been sug- 

 gested, they do not establish any other in the place of it. The only 

 general fact which I have noticed is, that in many instances, though not 

 in all, one of the two axes of elasticity which are perpendicular to 

 W, is also the axis of one of the principal zones of the crystal. 



3. To find the angle between a normal to any face {T) of a 

 crystal, and the apparent direction of one of the optic axes as seen in 

 air through any parallel faces of the crystal. 



Let the crystal be attached to an index, moveable on a graduated 

 circle having its plane parallel to the axis of the polarizing instrument, 

 or a table on which the position of the index may be marked by a 

 line drawn along its edge with a tracing point. Let the crystal be 

 placed in such a position, that the apparent direction of the optic axis 

 in air and a normal to T may be parallel to the circle. Move the 

 index till the center of the coloured rings coincides with a mark in 

 the axis of the polarizing instrument, and observe the points in which 

 it meets the circle. Turn the crystal half round in the plane of T, 



