ment of points in space — namely, the symmetry of the crystal- 
lographic systems and the law which governs the inclinations 
of the faces (law of rational indices). 
There are, however, subdivisions of the various systems 
consisting of the merohedral or partially symmetrical crystals 
belonging to them, which are not explained by the geometry 
of a network ; these consequently were referred by Bravais, 
not merely to the arrangement of the molecules in space, but 
also to the internal symmetry of the molecule itself. 
Hence the theory of Bravais, while able to a certain extent 
to explain the form of crystals, requires an auxiliary hypoth- 
esis if it is to explain those modifications which are partially 
symmetrical or merohedral. 
Sohncke,' treating the problem in a different manner, and 
reasoning from the fact that the properties of a crystal are the 
same at any one point within its mass as at any other, but dif- 
ferent along different directions, inquired in how many ways a 
system of points may be arranged in space so that the config- 
uration of the system round any one point is precisely similar 
to that round any other. Such a configuration may be called 
a Sohncke system of points in space {regelmdssiges Funktsys- 
tem). 
From his analysis of this problem, it appears that there are 
» " Entwickelung einer Theorie der Krystallstruktur." (Leipzig, i879)- 
