(423 ) 



not infinitely narrow. The measurements were made by means of 

 a microscope made by Zeiss, object-glass D, compensation eye-piece 

 6 with micrometer; also low powers were used. 



The above mentioned small holes in one of the slit sides gave 

 on 5 and 8 circular and on the other plates — on account of the 

 height of the X ray source — elongated dark images (cf. fig. 1 and 2). 

 On all plates the distances between the centres of these images 

 were divided with the dividing engine in the same number of equal 

 parts. Whenever for a definite division in one of the diffraction 

 images the corresponding width of the diffraction slit was wanted, 

 we could by this way immediately take it from the measurements 

 of 5 and 8. 



If, using small power — object-glass a*, index at 10, compen- 

 sation eye-glass 6 — plate N". 2 is gradually displaced, one sees 

 that the image of the slit appears in the broader part as a black 

 line, dark in the centre and with hazy edges, that however in the 

 narrower part the darker part — nucleus — ceases and the image 

 of the slit broadens out somewhat like a plume, showing but little 

 difference of intensity in a direction perpendicular to the length 

 of the image. In accordance with the gradual narrowing of the dif- 

 fraction slit, the image becomes of course weaker, but narrower 

 alone at a few places, in such a manner that from the point, where 

 the nucleus seams to disappear, maxima and minima begin to appear 

 in the width of the image. 



The same thing occurs also in the other negatives, most clearly 

 in W. 2, 6 and 9. 



In order to give some idea of the character of these faint broadenings, 

 which are observed at their best with the microscope, we have made 

 of the narrowest part of 5 and 2 enlarged photographs by means of 

 a so called mikroplanar N"'. la made by Zeiss. Figures 1 and 2 

 are reproductions of these enlargements by means of heliography. 



Fig. 1 is enlarged 16 times, fig. 2 14 times. 



Identical numbers at the divisions indicate corresponding places. 

 In fig. 1 is given also the width of the diffraction slit. 



In order to be sure that the real cause of the described pheno- 

 mena is to be attributed to diffraction, we have carefully consi- 

 dered other different causes whicli might cause a broadening of the 

 image of the slit. It appeared however, that though photographic 

 irradiation, local differences in the sensibility of the photographic 

 film, secondary rays (Sagnac), small motions of the stands durino- 

 the experiments, often extending over more then 10 days, may have 

 changed in different degrees the image of the slit, these causes 



