( 250 ) 
could be deduced without difficulty, which part of the strip was 
situated in the direction of the passage of the second slit. It was 
now easy to place plate IT with its piece of angle-iron in such 
a way — the holes in the iron were somewhat larger than the 
diameter of the screws — that, seen from the centre of the second 
slit, the part of the strip, just now determined, appeared exactly 
above the first slit. Plate I being able to revolve round an axis 
through the first slit, the latter could be directed in such a way that 
the rays from a Röntgen-tube placed near « fell on the second slit ; 
by means of a fluorescent screen we could make sure that this had 
been obtained. During the course of the diffraction experiments itself 
the exact position of the tube was several times controlled and if 
necessary corrected. 
The width of the second and first slits were arrived at from 
photographs when the photographie plate was placed immediately 
behind plate I and the Röntgen-tube at « or the photographie plate 
at « against plate I and the Röntgen-tube at 4; the photograph of the 
second slit was taken both before (namely April 10%, plate N°. 1) 
as after (namely Aug. 23°¢, plate N°. 2) the experiments. 
As has been mentioned before the self-regulating tubes with water- 
cooling were exclusively used; how well these tubes work and how 
excellent they are for the usual medical purposes, for the uncommon 
demands of this investigation only a few of them could be of ser- 
vice. For we wanted tubes which were “soft” and remained so for 
hours at a stretch, whilst the effect was so great that the cooling- 
water kept on boiling; most of the tubes became harder after a ten 
hours’use; when the discharges took place to the sides of the leaden 
case another tube had to be taken. 
Three very good photographs were obtained, to be distinguished 
as A, B and C. 
A, obtained on May 7 and 8 after an exposure of 9 hours and 
a half, principally by a very excellent tube furnishing very strong 
rays and of great softness; developed during three quarters of an 
hour in 200 eem. of glycine *) 1 to 5. 
B, obtained on July Sth, 9%, 10%, 12; time of exposure 31 hours; 
two tubes were used, one of which was soft for four hours 
and after that became hard and the second continually hard ; 
developed in one quarter of an hour with glycine 1 to 5. 
1) Voget’s Taschenbuch, 1901, pg. 128. 
