921 
clear that the maxima of emission-intensity can produce only cone- 
shaped surfaces, of which the top can be in the secondary source 
of emission only. The angles at the vertex of these cone-surfaces 
may be of different size and the direction of the secondary emission 
may be adverse to that of the primary rays. If a thin bar, composed 
of compressed crystal powder, on which Röntgen light falls vertically 
in the direction of the axis-and which has been placed cylindrically 
in such a way that the axis of the bar') and that of the cylinder 
are in the same place, and only an opening remains in the film to 
admit the primary light-rays, the photographic image will show a 
profile section of the cylinder with the different cone-surfaces of 
maxima-emission-intensity. 
The vertex angles of the cones, and consequently also the distances 
between the lines in the film depend upon the erystal-form of the 
material in which the light falls, and of the nature (wave-length) of 
the ‘homogeneous) primary light-emission. A material when shone 
upon by homogeneous Röntgen-rays of small wave-length will show 
a different interference-figure from that which we obtain when it is 
‘shone upon by Röntgen-light of greater wave-length i. e. when the 
lightwaves are longer the lines will not be so close together. 
DeBye and ScHERRER were now able, whenever the wave-length 
of the primary homogeneous Röntgenlight was known, to infer the 
crystal-form of the material from the interference-figure obtained 
through the light falling in a bar of erystal powder or quasi amorph- 
ous material. 
Besides the many interesting, purely physical questions to which 
this highly important discovery will lead, the question offering itself 
in the first place from a chemical point is: What will be the result 
when. allotropic modifications of the same material are under the 
light of homogeneous X rays of the same wave-length; — and will 
it be possible to bring to light the different modifications that have 
been observed in the same material in a purely physical way. 
Here we think especially of the dynamical allotropies. 
The questions which we asked ourselves as soon as we had read 
Prof. DerByw's article, we have laid before the professor and it appeared 
that Prof. DesBre had” also considered this question, but that, 
owing to the want of suitable material, he had not made any in- 
vestigation in that direction. He, too, expected that in this manner 
it would be found possible to distinguish between allotropic forms. 
With the greatest willingness he left the investigation in this direc- 
') The shape is not of much importance. 
Proceedings Royal Acad. Amsterdam. Vol. XIX 
