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secondly interference fringes in the direction of the motion are seen 

 very distinctly. 



If the ocnlar is moved in an arbitrary way, bnt so that continu- 

 ally it remains sharply focnssed on the light-point in the leaden 

 screen, interference fringes are seen rotating on a homogeneous 

 white spot. 



This experiment cannot be made with a screen with a very great 

 number of apertures. Probably this is caused by the fact that in this 

 ease the number of interference fringes which have the same direction 

 becomes so great. The distance between the parallel fringes is 

 generally very different, by the superposition only the central fringe 

 will therefore remain and a set of [)arallel interference fringes in 

 the direction of motion will not be seen. 



The second experiment that will be described was suggested to 

 me by Prof. H. A. Lorkntz. 



The reasoning leading to it was the following: when with purely 

 monochromatic light the distribution of the intensity is granular, the 

 fibrous character of the diffraction image will be due to the spectral 

 shift of the grains from the violet towards the red. But when now 

 this spectral shift, which forms part of the nature of the phenomenon, 

 is the only cause of the formation of the tibres, we may expect 

 that by a second artiticial s|)ectral shift the fibres will be no longer 

 radially directed. We have really succeeded iji producing tibres in 

 other directions. When before the objective of the telescope, between 

 this lens and the diffracting plate we place a prism'), all diffraction 

 images from the violet towards the red will be shifted in a definite 

 direction, the red image over the gi-eatest distance. The light-point 

 at the centre of the circular diffraction figure, the image of the 

 aperture in the leaden screen, is changed into a light-strip AB, red 

 at the end A and blue at the other end B. If we consider the thus 

 formed diffraction figure on the production of AB on the side of 

 B, we shall reach a point J* where the original spectral shift belong- 

 ing to the diffraction image is neutralized by the sui>erposed spectral 

 shift due to the prism. In the neighbourhood of this point the 

 structure must be granular, while the newly formed fibres at some 

 distance from this point must be directed towards it. These pheno- 

 mena can easily be observed. So this experiment too proves that thé 

 fibres are due to a spectral shift. 



They are most easily observed when a well-chosen WRATTEN-filter 

 is kept before the eye; with such a plate, 'through which of the 



^) Also a small prism may be held between the ocular and the eye. 



