COLLECTION OF EXAMPLES. 



369 



section, diametral longitudinal sections or marginal views, and 

 surface views of cylindrical struc- 

 tures, we will employ a terminology 

 agreeing in principle with that applied 

 above (p. 356), though in so far differ- 

 ing from it that for transverse and 

 longitudinal sections (and with spheri- 

 cal objects for any sectional views 

 whatever) it is based upon the relation 

 to the course of the layers, not to the 

 diameter. We shall hence denote by p 

 (parallel) and by t (transversal) the 

 positions of the ellipses of elasticity, in 

 which their major axes lie respectively 

 in the direction of the layers, or cut 

 them at right angles. This relation to 

 the course of the layers is, moreover, not 

 arbitrarily chosen, but derived directly 

 from nature, inasmuch as the ellipses of 

 the known objects in longitudinal and 

 transverse sections, almost without ex- 

 ception, show the same position to the 

 stratification. The position of the 

 ellipses represented in Fig. 209, for 

 example, would consequently be, de- 

 noted as follows : transverse section 

 p, margin p, surface p. When diametral longitudinal sections 

 were examined in place of the marginal view, the term " longi- 

 tudinal section," instead of " margin," has been employed. The 

 two orthogonal positions of the object, which serve to distinguish 

 the consecutive and the alternative positions, according to what 

 has been stated above, are represented by horizontal ( = ) or 

 upright ( || ) strokes, in which, of course, the same position of the 

 selenite plates, and the usual position of the polarising prisms 

 corresponding to these strokes, are throughout assumed. Since the 

 optic axes always lie in the planes of the major and minor axes 

 of elasticity, it has been thought superfluous to specially name the 

 position of the axial plane. On the other hand, the positive or 

 negative constitution is given with the few objects which have 

 supplied the necessary data. 



