1201 
Physics. — “The Symmetry of the Röntgen-patterns of Monoclinic 
Crystals.” By Prof. H. Haca and Prof. F. M. Jancer. 
(Communicated in the meeting of Jan. 29, 1916). 
§ 1. In the following pages we publish the results, which were 
obtained by radiating through planeparallel plates of monoclinic crystals. 
As is well known, the monoclinic system possesses three classes 
of symmetry. The crystals of the sphenoïdical class only have a 
polar binary axis as their single symmetry-element; those of the 
domatic class only a single symmetry-plane; and those of the pris- 
matic class are in possession of a plane of symmetry, a binary axis 
perpendicular to it and a symmetry-centre. According to the now 
adopted theory of the diffraction-phenomenon here considered, a 
symmetry-centre is added to the elements of symmetry of the crystal 
already present. And just because the simultaneous presence of each 
pair of the three elements of symmetry here to be considered, 
necessarily involves the presence of the third of them, the crystals 
of each of the three classes mentioned will thus behave with respect 
to the symmetry of their Röntgen-patterns, as if they belonged to 
the monoclinic-prismatic class: for here are the three elements of 
symmetry mentioned above, exactly combined. Thus, indifferently 
to which of the three said classes a monoclinic crystal will belong, 
its Röntgen-pattern for a section parallel to. {100} or {O01} must 
always show one single plane of symmetry, while for a section 
parallel to {O10} only « single binary avis, perpendicular to the 
photographie plate will manifest itself. By the investigations described 
in this paper, we have brought full proof of the exactitude of these 
conclusions. 
§ 2. As representatives of the different classes of symmetry, we 
have studied the following crystals: 
a. Of the monoclinic-sphenoidical class: dextrogyratory tartric acid, 
cane-sugar and dextrogyratory rhamnose. 
b. Of the monoclinic-domatic class: scolezite. 
e. Of the monoclinic-prismatic class: gypsum, epidote, amphibole 
and augite. 
I. Monochnic-sphenoidical class. 
Plates parallel to {100}, {010} and {0015 were cut from some 
beautiful, transparent crystals of dextrogyratory tartric acid: C,O,H,, 
whose axial ratio is: a:b:c=1,2747: 1:1,0266, and 8 = 79°43. 
The erystal-plate parallel to (100) had a thickness of 1,08 mm., 
that parallel to (010) of 1,15 mm., and the plate parallel to (001) 
of 1,18 mm. 
