( «^7 ) 



as their place, as deduced lioin the theory of those diffractiou 

 images, is not quite iudependeut of wave-length. We now easily 

 understand this particular to be also due to the optical illusion. 

 The differences of colour, which should really exist to some extent, 

 have intensities, which correspond to the real excesses of ///«/«//*«/ /om 

 in the principal maxima and by no means to the much greater 

 excesses of brightness^ which our eye perceives there. 



20. Whereas our optical illusion leads in this way to tlie expla- 

 nation of some difiGculties, it compels us on the other hand to give 

 up our former conclusion (cf. Art. 15), that the analogy between the 

 shadow images of X-rays and of ordinary light justifies the supposition 

 that the X-rays consist of undulations. The facts mentioned in my 

 former communication sufficiently show, that even without a trace of 

 diffraction the X-rays could produce shadow images, which present to 

 our eye the well-known bright and dark fringes. The photo a repro- 

 duction of which is given in that communication as fig. 6 shows in 

 originali a character which agrees perfectly with the (negative) X-shadow 

 images, obtained formerly by FojiM and others. Its dark fringes are 

 by no means less distinct than those of the X-shadow images men- 

 tioned ; and on the other hand the latter show very distinct bright 

 fringes on their outer sides — though there may have been paid 

 little attention to these fringes hitherto — , these bright fringes being 

 exactly those corresponding to the bright circular lines in fig. 6. 

 Moreover it is by no means difficult — as I pointed out in Art. 8 

 of my former communication — to obtain the X-shadow images 

 with the characteristic bright and dark fringes by using slits 

 of a considerable width (cf. fig. 10 of that communication). This 

 however would contradict with expectations founded on the suppo- 

 sition of those fringes being caused by diffraction. 



21. The optical illusion alluded to leaves, of course, beyond all 

 doubt the correctness of the theoretical considerations, communicated 

 before, on the influence of the widening of the luminous slit on 

 diffraction phenomena. Especially the simple method, developed in 

 order to take that influence into account, holds quite good as far 

 as concerns an exact interpretation of the diffractiou images observed 

 and a calculation of the distribution of light, which may be expected 

 in diffraction images under given conditions i). 



1) It is easy to see that my method for calculating the influence of the widening 

 of the slit on the distribution of light in difi'raction images also applies, in principle 

 at least, to other difi'raction phenomena than those of Feesnel, e. g. to those of 

 FRiUNHOFEH, and it is easy to conceive how this method is to be modified for 

 being applicable to such cases. 



5 



Proceediugs Pioyal ,\cad. Amstcrdaui Vol. I. 



