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case (thickness of the side 2 mm.); only the back of this case was 
left open for the connecting wires of the induction coil, whilst in the 
front a small aperture was made opposite the first slit to let the 
Röntgen-rays pass. The first slit was formed by two platina plates, 
thick 2 mm. and high 2 cM; a leaden sereen left but the middle of 
it free over a height of 4 mm.; the width of this slit was 15 u. 
The diffraction slit was formed by two platina plates thick */, mm., 
high 4 em., tapering at the upper end from a width of 25 u to 
nearly zero at the other. Greatest care was given to the grinding of 
the platina plates. For these slits, just as in our former experiments, 
the sides were everywhere equally thick and not ground wedge-shaped 
at the edge of the slit as is the case with slits for light-experiments. 
To produce the Röntgen-rays an induction coil of Siemens and 
Harskr was used with a sparklength of 60 em, a primary with 4 
coils and a Weanevr-interruptor. The current was furnished by a 
battery of accumulators of 110 Volt. The newest Röntgen-tubes of 
Mitimr (Hamburg), were exclusively used; the anticathode being 
kept cool by water. 
More care than formerly was taken to bring the passages of the 
two slits accurately into the same line. Peculiar difficulties are incident 
to this, which is a result of the extraordinarily great depth of the 
slits together with a width so slight that common light, on account 
of the arising phenomena of diffraction, cannot be used to fix accurately 
the direction of the passage. For this last reason we had to have 
recourse to Röntgen-rays to do so. And here again the slight width 
of the slits caused the pencils of rays passing through them to be so 
very faint, that in the ease of the jist slit, wide 15 u, they could 
be observed on a fluorescent sereen at the place where it was 
necessary, namely near the second slit only with an eye accustomed 
to complete darkness. The pencil of rays, allowed to pass through the 
most interesting part of the second slit viz. that part, where the width 
was about 5 u, could be observed not even by this way, but only by 
the impression it made on a sensitive plate after a lengthy exposure 
(4 hours). In order to deduce from this impression a mark for the 
direction of the passage of the second slit, a small strip of brass was 
fastened near and slightly above the first slit; (see figure); in this strip, 
held by an arm fixed to plate IL, some vertical rows of small holes 
had been drilled side by side, differing in number and size. A Röntgen- 
tube was placed behind plate IL in 4 and a photographie plate between 
the brass strip and plate I; a small leaden sereen left of the second 
slit but the part to be observed free. On the photographie impression 
one or two of the rows of the holes became visible and from this 
