( «30 ) 



at least a deterniinatidu of a possibly low upper limit for such a 

 wave-length. With a view to this purpose Prof. Haga and myself 

 have continued the experiments begun by Mr. Tiddens ; in the 

 beginning, however, always with a negative result. 



Finally two experiments have been made these last days under 

 iW following conditions. In both of them the width of the luminous slit 

 was ty = 49 // and this slit received the radiation of an X-ray tube 

 (with self-regulating vacuum according to the newest system of MCller 

 at Hamburg), placed in such a way that the anticathode made a 

 very small angle with the axis of the arrangement (cf. Art. 1) in 

 order to procure the greatest possible concentration of the pencil of 

 X-rays, terminated by the slit. 



One of the ends of the diffraction slit was narrower than the 

 other, the latter being of a width of ± 400 u, the other of a few 

 microns only. In the experiment A the distances were: « = 293, 

 b = 29S cM., in the experiment B: a = 605, b = 615 cM. 



Both the slits as well as the sensitive plate were attached to 

 firm -stands, fastened by means of plaster to columns of compact 

 lime-stone, which were attached with plaster to the pillars or the 

 stone floor of the building. The time of exposition was a little 

 more than 8 hours for the experiment A, 40 hours for the expe- 

 riment B. 



After development each of the two plates clearly showed a thin 

 black line, sharpening towards one end and being the image of the 

 second slit. 



By first observation there is no influence of diffraction to be 

 perceived on either of the plates, the borders of the black line 

 mentioned appearing almost absolutely sharp defined and the line 

 itself ending in a very sharp point. Closer observation, however, 

 shows on both sides a transition zone limited by a dark line on the 

 inner side, by a bright one on the outer side. Finally, by looking 

 carefully at the lines under a microscope, magnifying about 14 times, 

 I have been led to the following observations. 



1°. On plate A the dark and the bright lines are pretty sharply 

 defined, much like those appearing in transition zones, where there is 

 no diffraction. The distance between the dark and the bright fringe 

 has for each of the transition zones an average value of about 67 // 

 over nearly the whole plate, while on the other hand the width of 

 the transition zones as calculated from the values of g, a and b 

 would amount to 50 ,w, in the supposition of rectilinear propagation. 



Comparing the plate with the series of images mentioned in 

 Art. 21. one may feel inclined to assume that the difference between 



5* 



