545 



The dimensions are chosen in snch a way, tiiat the dislaiice from 

 the front face of tlie cover lo Ihe sensitive film in /-' is precisely 

 50 m.in. ; of course the thickness of tiie fluorescent screen and of 

 the two black paper covers, with which the plate and screen are 

 protected, are taken into account here. 



The photographic plate, with the fluorescent "Eresco"-screen pressed 

 against the sensitive film, was wrapped in two covers of black paper 

 and then firmly pressed against P, it had an opening measuring 

 8X8 cm., and the whole apparatus thus was held together much 

 in the same way as in the case of a photographic copying-press. 



The three plate-bearers D could be adjusted into the right position 

 with sufficient accuracy by means of three straight, thin knitting- 

 needles, which after being pushed through the canals in the lead 

 cylinders, must meet in the same point J. For the purpose of making 

 the anti-cathode coincide with this point A, the wooden bearer of 

 the RöNTGEN-tube was fixed on a heavy brass support, which had 

 smoothly running sliding-motions in three perpendicular directions; 

 thus it was made possible, to fix the RöNTGEN-tube exactly in such 

 position that the three pencils of RoNTGEN-rays generated three equally 

 strongly lundnous little spots on a fluorescent screen, which was 

 placed behind P. In the plates P three central holes of 1 cm. dia- 

 meter were bored to enable us to see these luminous spots. To protect 

 the photographic plate against undesired attack by direct or secondary 

 RöNTGEN-rays, some larger lead screens were interposed between the 

 RöNTGEN-tube and the plate-holders with a total thickness of 2 cm.; 

 in the same way the three crystal-, and plate-holders themselves 

 were surrounded by a lead cover, which could be closely fitted to 

 the large lead screens. In the backside face of the lead cover three 

 holes were boi'ed, big enough to let the undift'racfed RöNTGEN-rays 

 freely pass. 



An inconvenience, met witii in our former exi)eriments when using 

 the fluorescent screen, was the abnormal sizeof Ihe central spot on the 

 photos, which spot would even seem still larger in the reproductions 

 from the negatives '). The extension of this spot must be caused by 

 the action of the secondaiy RoNTGEN-rays, which were produced by 

 the passing of Ihe undillVacted pencil through the glass and the 

 sensitive film; these secondary rays will provoke a rather strong 

 fluorescence of the vicinal parts of the screen and thus an intense 



') The diameter of tlie image of the undiffracted rays was aliout '1 m.m., as 

 can also be calculated from the used dimensions of our apparatus: by photographic 

 irradiation or by the mentioned secondary rays however, the central spot on the 

 photos appeared to be about 8 m.m. in some cases. 



