1284 



systems are effaced, and not only this, but tliej even vanish totally, 

 the mean value of cos{t„ —<Pt) being zero. As now the distance 

 between tlie lines a, h is inversely proportional to the distance a, h 

 every line-system correspondiuj^- to apertures or grains at a consi- 

 derable distance from each oilier will vanish. It remains however 

 questionable whether this vanishing of a number of line-systems 

 increases the visibility of the others. 



7. The phenomena may also be treated in a different way 

 (though of course equivalent with the former). We maj namely 

 calculate first the total resulting distribution of intensity i for one 

 wavelength, and then superpose all these distributions for the different 

 wavelengths {Si). Doing this, we must keep in mind (hat the same 

 intensity which for a wavelength / occurs at a point /^ is found 

 for another /' at a point /^' which lies with and P on a straight 

 line in such a way that OP: OP' ^ ).-.)'. 



According to the classic theory the distiibution of light i is 

 "granular" for a definite ).. If we pass to another wave-length X' , 

 the light and dark spots are shifted in radial direction. This will be 

 called "spectral shift". 



It is evident that if we pass from monochromatic light to 

 homogeneous light the grains will iji this way be changed into fibres. 



ExNKR ') thought that the observed fibres could be sufïiciently 

 explained in this way. Von Lauk combatted this and drew the 

 attention to one of his photographs of the diffraction figure of a 

 plate covered with lycopodium powder on which the fibres in the 

 first diffraction ring are longer than would correspond to the 

 spectral shift. 



(See also the end of § 5). 



8. In my view v. Lauk has paid here no attention to the fact 

 that by the running into one another of the light spots and also of 

 the dark ones, there may be formed fibres, longer than would 

 correspond to the elongation of each separate spot. Let us consider 

 different lines starting from 0. Because of the accidental distribution 

 of the light and dark spots (in consequence of the accidental distri- 

 bution of the lycopodium grains) the distances between the dark 

 spots will be somewhat smaller on one radius than on another. 

 At a certain distance from the ce'ntre we shall sooner see a 

 somewhat dark line along one ladius than along another. In this 

 way the fibres would be formed. The question is now: 



') Sitzungsber. Akad. Wien 76 (1877), p. 522; Ann. d. Phys. 9 (1880), p. 239. 



