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scctino'. According to theory there should be soon after the point 

 of intersection an area of nearly uniform maximal illumination in the 

 middle of the diffraction image, covering the whole region between 

 the two places wliere in fact the maxima are seen. This difficulty is 

 solved by the optical illusion, v^hich I liave described in the Procee- 

 dings of the May-Meeting. In cases like that mentioned — viz: those, 

 where a zone of uniform maximal illumination gradually passes on 

 both sides into zones of continuously decreasing illumination — there 

 are indeed two separate maxima to be observed on the borders of 

 the bright zone (of. fig. 9 and 10 in the former communication). 



18. This proves already that the optical illusion mentioned may 

 largely influence our observations of diffraction phenomena. A.S also 

 appears from some experiments, which I have lately made for this 

 purpose, this influence may even be so great, that the real distribution 

 of light in the diffraction images can hardly be deduced from imme- 

 diate observation in many cases. So, for instance, it has struck me 

 repeatedly that the diffraction image of a slit still shows the principal 

 maxima with great distinctness, even when the luminous slit has a 

 considerable width, whereas theory indicates the excess of intensity 

 in the maxima, compared with the field round them, to become 

 smaller and smaller, when the slit is widened more and more. As 

 we know now, that under the influence of the optical illusion 

 distinct maxima of brightness are observed even on the borders of 

 a zone of uniform illumination, provided that it passes into zones of 

 decreasing intensity, we may by no means wonder, if we see the prin- 

 cipal maxima apparently continue to be clearly visible, though their 

 excess of intensity become very small. We may ask however, 

 whether, because of the optical illusion, the place where we observe 

 our maxima of brightness be still the same as the place where those 

 maxima of illumination leally occur. And without any doubt the 

 answer ought to be: no. Yet it may be presumed and experiments 

 have confirmed, that, as long as the principal maxima themselves 

 are of a considerable distinctness (excess of intensity)^ their apparent 

 place is only slightly influenced by optical illusion, that i. o. w. 

 the place, where our eye observes a maximum of 6?7'(//isi«ess, is chiefly 

 determined by the principal maxima themselves. This explains why 

 some measurements of secondary diftVaction images, mentioned in 

 my second comnuinication on the subject, may have led to a taxa- 

 tion of the wave-length of light which was not quite wrong. 



19. Another remark relating to my former experiments is this. 

 The maxima observed in the secondary diffraction images are of a 

 pure white, though we should expect them to be slightly coloured, 



