1282 



is used the inaxiinnm and ininimiun lines cover the nihole Held of 

 view. It is to be expected that a great number of such line-systems 

 crossing each other in all possible directions give a "granular" structure. 

 This would be the granular structure, whicli according to v. Ijauk's 

 theoretical considerations must be observed with homogeneous light. 



The question, whether the classic theory can explain the observations 

 of v. Laue would be answered immediately, if the experiment could 

 be made with really homogeneous light. As to the central spot, this 

 may be done, but because of the small intensities it is already very 

 difficult for the first ring. From our different experiments some will 

 be described : 



1"^. By the aid of Wratten filters the light was first more or less 

 monochromatized. Here as in the following experiments the three 

 different screens were used, that with the apertures, the glass- 

 plate with lycopodium and that with mercury. The fibrous structure 

 at the centre of the central diffraction spot vanished and became 

 granular. At a greater distance from the centre the fibres in the 

 central spot remained. It is interesting to remark that the length of 

 each fibre does not increase proportionally with the radius. The 

 fibres near the periphery are relatively much longer than those nearer 

 the centre of the spot. 



2"*^. The sodium light-source was used. Now the central spot was 

 beautifully granular even at the periphery. Of the first ring a weak 

 shadow was observed. It is difficult to say whether it contained 

 anything radial. The sodium light was formed by the flame of a 

 BuNSEN burner in which a spoon with sodium was held. Strong light 

 and a high temperature of the flame were avoided, though by means 

 of these a very intense sodium-light can be obtained, as e. g. du Bois ') 

 described. This was done in order to obtain higher monochromatisy. 

 The light-source being therefore weak the observations were not 

 made with a telescope, but with the naked eye. The glass-plate 

 was fixed close to the eye which instinctively was accomodated to 

 the illuminated aperture. 



The observations were made in an absolutely dark room. The 

 flame was placed in a perfectly closed chest of iron with a communi- 

 cation tube to the air. Along a fixed sliding the spoon with sodium 

 could be brought into the flame, so that never any light except 

 through one single fine aperture left the chest. 



6. With natural light the phenomena are quite different from 

 those observed with homogeneous light. Instead of (2) we then must 



^) Du Bois. Zeilschr. f. Instrumenieukunde, 1892, p. 165. 



