407 



fell u[)on the film, as indeed Dkbijk and Sciierrkk aireadj- did too. 

 The couiiiig up of tlie second i<ind of nndesired lines was avoided 

 by extracting (he lead screen a little from the surrounding brass 

 tube, so (hat (he rajs coming from the lead, were screened from 

 the camera by the remaining wall of (ho camera, or by widening 

 the end of the screen, which was directed to the film so much, that 

 the rays, emerging from (he back edge of the not-widened part of 

 the screen, were arrested by (he widened |)art of it. ^) 



On the photos, obtained by us of (he whi(e, as well as of (he 

 grey tin thei-e appear points on (he lines, as were also observed by 

 Debuk and Scherker on (heir photo of silicium. They must be 

 ascribed to largei- cry stal -pa reels. '^) 



In order to facilita(e the compai-ing of (he resid(s for whi(e and 

 grey tin, we have indicated in (he drawing undeineadi, one under 

 the other for both these materials, by vertical lines the places, where 

 the interference-cones intersect the film in a plane, perpendicular to 

 the axis of the preparation. The Jiumbers at (he bo((om give the 

 distances of the lines to the intersection of the axis of the penetrating 

 beam with the film in mm. By greater or smaller thickness of 

 the lines at the top and the bottom respectively (he in(ensi(y of (he 

 lines is given, as it was estimated by us in five degrees, namely : 



^) Perhaps it is worth while, to state, that the criticism, exerted by Taylor, 

 Physik. ZS. 17 (1916) p. 316 on articles of Laub, Pliysik. ZS. 15 (1914) p. 732, 

 844, seems not to be founded, now that we too found, that characteristic X-rays 

 when going tlirough a leaden screen, can give interferencelines, originatmg from 

 the edges of the screen. 



') The length-direction of these oblong points is the same as the direction of 

 the interference lines. Tl)e explanation of this is as follows. On a single point of 

 a plane out of the atom-net of such a crystal-parcel there falls, on account of the 

 imperfect parallelism of the beam, which has passed the screen, a convergent 

 conical beam of rays (fust cone). If the net-plane, mentioned, has such a position 

 that for a direction of rays in the axis of this last beam the condition for favourable 

 interference is fulfilled, then there are in this beam still other directions of rays, 

 for which this condition is just as well fulfilled The collection of these directions 

 is found by describing a cone, with the normal on the plane mentioned, as axis 

 and the axis of the first cone as one of the describing lines (second cone). All 

 the describing lines of the second cone that are within the first cone, fulfill the 

 condition for favourable interference. After their reflection all these rays form together 

 a diametrically opposite part of the second cone. Just there however the 

 second cone is tangent to the third cone, formed by the reflected rays, which 

 are obtained by letting the reflecting plane take all possible positions favourable 

 for interference. Now, where the film intersects with both the last cones, the 

 intersection-lines will be tangent to each other. One of these intersection-lines js 

 an interference line, the other such an oblong point. 



