1190 
is put for shortness. 
Thus we get the dark rings: 
w(t ‚sn 2720 
220 
which were mentioned in § 1°). 
Determination of the mean value 11. 
The distances from Röntgen tube to molecule, and from molecule 
to photographic plate being enormously large in comparison with 
the distance AL=u, we could act up to now as if we had to deal 
with an interference problem of FRAUENHOFER. If for the determination 
of the mean value II the molecule is made to cecupy all situations 
in the irradiated space, these molecule displacements are practically 
still infinitesimal compared with the distance from Röntgen tube, 
but not compared with the distance from molecule to plate. 
In connection with this in the slight displacements of the molecule 
parallel to the photographie plate the dark rings move over an 
equally large distance. In displacement normal to the plate a slight 
enlargement or diminution of the rings occurs. We see that this 
brings about a slight fading of the rings. 
Geology. — “On the granitic area of Rokan (Middle-Sumatra) 
and on contact-phenomena in the surrounding schists. By Dr. 
H. A. Brouwer. (Communicated by Prof. G. A. F. MOLENGRAAFE). 
(Communicated in the meeting of June 27, 1914). 
Between Rokan and Loeboek Bandhara the Rokan Kiri cuts a 
granitic mass of about 4°/, km. in width, which on its northwestern 
and southeastern sides is adjoined by tertiary sandstones and conglo- 
merates, which however, along a portion of the south west limit, are 
separated from the granite by a narrow strip of schists. 
During the exploration of this area some facts were collected 
concerning the various facies of the granites and the contactphenomena 
in the surrounding schists. The schists dip towards the granitic mass 
1) The factor M? varies of course also with 9; perhaps in the same way as 
cos? g. In § 1 we have however disregarded this variation for the present to get 
a first survey. 
