( 421 ) 



All the apparatus could now be mounted on one plate of free- 

 stone, whereby moreover greater steadiness was secured. 



Tlie first slit (X slit), the second (diffraction slit) and the photo- 

 graphic plate were mounted on heavy metallic stands, resting on 

 a free-stone plate (200 X 40 X 30 cm.) This plate was supported 

 by three columns of the same material, resting on the great pillar 

 of the edifice; stands, plate, column and pillar were united by 

 plaster of Paris. 



Behind the first slit was the Röntgen tube, one of the excellent 

 tubes made by Muller (Hamburg) with automatic vacuum regu- 

 lator. 



The iuduction coil was an excellent piece of apparatus made by 

 Siemens and Halske, with a maximum spark-length of 30 cm. aod 

 a Deprez interrupter with two contacts. 



The current for the induction coil was given by 6 accumula- 

 tors. The tube and the first slit were surrounded at all sides — 

 except at the backside, where space was kept for the connecting 

 wires between the coil and the tube — with thick leaden plates; 

 in the direction of the diffraction slit a small aperture was spared 

 in order to enable the X rays to reach the photographic plate. 

 Much pain has been taken to obtain the slits as excellent as possible; 

 the small platinum plates, thick 1/2 mm., which formed the edges 

 of the slit, had been very carefully flattened and ground and were 

 screwed on flattened plates of brass. The width of the X slit was 

 14, 18 or 25 microns; by means of a leaden plate the height was 

 limited to 1 cm. 



The diffraction slit (height 3 cm.) was 14 microns at the upper 

 end and gradually narrowing to a width of a few microns. 



On one of the sides of the diffraction slit and very near to it 

 there had been drilled small round holes in the platinum, near to 

 the upper and lower end and to the centre 0' the slit, in order to 

 enable us, in a way afterwards to be mentir ned, to know the effec- 

 tive width of each part of the diftVaction slit. The diffraction slit 

 ended at the top and at the bottom in a slit of considerable width 

 (3 m.m), the axis of which was the prolongation of the axis of the 

 diffraction slit; the Röntgen tube was placed behind the first slit 

 in such a position that these wider slits cut from the pencil of 

 X ravs the most intensive, middle part, as could easily be controlled, 

 from time to time, by means of a fluorescent screen; of course the 

 rays then passed also in the desired manner by the diffraction slit. 



With this arrangement were taken the experiments of which the 

 results are given in the following table, where signifies 



