1206 
the plane, perpendicular to the binary axis, which disappears at the 
same time as the symmetry-centre. 
Thus in both these cases the symmetry of the RÖNTGRN-patterns 
will evidently remain unchanged ; they will show the same symmetry 
as the R6nreEN-patterns of the holohedrical forms of the system 
would possess. From this results that all merohedrical crystals whose 
symmetry can be derived in the ways described sub a) and ©), will 
give RönrerNograms of the same symmetry as in the case of the 
corresponding holohedrical crystals. 
Only for the merohedrical crystals of the type mentioned sub 5), 
the RörreeNogram will possibly manifest a different symmetry, than 
may be expected in the case of the holohedrical forms. 
The sub a) mentioned symmetry-groups are characteristic of all 
erystals, which only possess avia/ symmetry ; that is to say : for all 
crystals of those eleven classes, whose forms are different from their 
mirror-images, and which can exist therefore as enantiomorphous 
polyhedra. Thus all dextro- and laevogyratory antipodes will neces- 
sarily manifest identical RONTGEN-patterns. 
Furthermore to the groups derived sub a) and ec) will belong all 
those crystals whose symmetry is that of hemzmorphic crystals ; in 
the latter therefore the absence of the symmetry-centre will not be 
shown by the RörrceNograms in any other way than in the case 
of crystals of other symmetry-classes. 
On more detailed consideration it appears that the cause, why 
the absence of a symmetry-centre in the crystals can never be 
revealed in the RÖNrGeN-patterns, is to be ascribed to tbe fact, that the 
venerated secondary radiation is in itself of a centrically symme- 
trical nature, just as in the case of ordinary light-waves. If tbis were 
not the ease, then the symmetry of the RörrGeNograms could be 
discerned in the same 32 symmetry-classes, just as with the poly- 
hedrical erystalforms themselves, which are generated under the 
influence of the one-sided forces of erystallisation. However it will 
appear that even such a supposition would not be sufficient to give 
an explanation of the new phenomena to be recorded here. 
$ 2. The problem we have had before us for a long time, and 
which evidently could only be answered by means of numerous 
experiments, was just this: what symmetry will ev entually be revealed 
in the RöNreeN-patterns of all kinds of merohedrical crystals. 
Originally it seemed as if the experience hitherto obtained fully 
supported the correctness of the above mentioned theoretical de- 
ductions. 
