( 12 ) 



The image on the screen will show very beautifully the bright 

 and dark lines mentioned in 1. On photographs taken of' this image, 

 these lines are very clearly visible too ; fig. 9, a reproduction of 

 such a photograph also shows them, but much less well-defined. 



If the second slit is sufficiently narroiuer than the first, the image 

 on the screen will not show a part illuminated by the whole of the 

 first slit, yet there will be again a middle band of maximum 

 illumination across its whole extent; and for the rest the image on 

 the screen will show as a whole the same characteristics as in the 

 preceding case as to both real and apparent brightness. If the second 

 slit is gradually narrowing towards the lower part we see an image 

 projected on the screen in which two straight bright lines appear, 

 these lines, parallel with the edges of the second slit, intersecting so- 

 mewhere, but remaining clearly visible beyond the point of intersection. 



If we cause the light emitted from the first slit to cast a shadow 

 of a thin needle or thread, we get a silhouette with a middle zone 

 having over the whole of its breadth uniform minimum brightness 

 bounded by transition zcmes of uniformly increasing brightness 

 which on their outsides again are bounded by areas of uniform 

 maximum illumination. The maxima, and especially the minima 

 alluded to in 1. are again very clearly visible here, even to such a 

 degree that in some cases the appearing of the minima might lead 

 one to speak of a doubling of the shadow cast by the thread. 



In all these cases, if only the slits arc wide enough, diffraction 

 plays no perceptible part. 



8. If we illuminate the first slit of 7. by a X-ray tube instead 

 of by ordinary light, and if the rays are not caught on a screen 

 but on a sensitive plate we get on developing, negatives of which 

 the positives are exactly simih^r to the images described in 7. 

 Fig. 10 shows a reproduction of such a positive, corresponding to a 

 similar case as fig. 9. 



Fig. 10 moreover shows a white rectangle, which covers part of 

 the drawing. This effect was obtained by covering part of the 

 negative, of which it is a reproduction, with a slip of paper during 

 the copying. I did so in order to point out that the disappearing 

 of the transition zone — which was effected on the spot in ques- 

 tion, at least for the greater part, by this slip of paper — is suf- 

 ficient to cause the line corresponding to it to vanish. Indeed the 

 bright line which in the other parts of the image is still visible, 

 however mucn it has lost of its clearness by repeated processes of 

 reproduction is no longer to be traced where the tronsitionzone 

 has been covered, which sufficiently proves the fictitious nature 



