1185 
the decrease of darkness. Frmepricu shortly discusses two possible 
explanations of the observed phenomenon. 
a. The solid amorphous bodies may be imagined as built up of 
small erystals. The interference spots of the different individual 
bodies uniformly turned in all directions unite on the photographic 
plate to rings. 
6. For the solid amorphous substances and particularly of the 
liquids is “die Anordnung der Teilehen eine vollkommen regellose”’. 
In the case of the passage of Röntgen rays through such an amorph- 
ous medium we should meet with a phenomenon analogous to that 
of the passage of rays of light through a glass plate strewn with 
lyeopodium powder. ') 
Friepricn sites the circumstance that also for liquid paraffin an 
interference ring occurs, in favour of the second explanation, and 
he therefore expresses the supposition that here we should have to 
do “mit Beugung am Molekiii resp. Atom”. 
E. Hupka’*) tries, if I have understood him correctly, to give 
another explanation, in which the mean distance of the molecules 
is decisive as “grating constant”. 
A trustworthy introduction of statistical considerations on which 
the calculation of the dark rings must rest, does not seem easy to 
me even for ‘liquids on account of the compact arrangement of the 
molecules and particularly on account of the unknown complexes 
(association) of adjacent molecules. 
I may be therefore allowed to point out briefly that the problem 
is considerably simpler in case of transition of Röntgen rays through 
a di-atomic gas. Whether the experimental difficulties can be sur- 
mounted, I cannot judge; in case this should be so, some new data 
might be obtained in this way on the situation of the atoms in the 
gas molecule. 
$ 1. Let homogeneous plane Röntgen rays fall on an isolated di- 
àtomie gas molecule. Both atoms emit secondary waves which inter- 
fere in the whole space. We consider the interference in an arbi- 
trary point P of a plane / (photographic plate), which lies normal 
to the direction of incidence of the Röntgen ray at the distance D 
behind the molecule. ) may be considered as infinitely great with 
- 1) Drupe, Optik I, Afd. Kap. IV; M. Lave, Beugungserscheinungen an vielen un- 
regelmässig verteilten Teilchen. Sitzber. d. preuss. Akad. 1915. 
*) E. Hurka, Die Interferenz der Röntgenstrahlen Samml. Vieweg, Heft 18 
1914), p. 62. 
78* 
