1278 
approaches less rapidly to zero than that in the direction perpendicular 
to it, whereas we have also to take into account that the distance 
between the source of radiation and the point of observation is not 
infinitely great with respect to the dimensions of the plane struck 
by radiation. Trying to explain the form of the spots by assuming 
a rectilinear propagation we do not come to the right result. E. g., 
if we have to do with a reflecting plane lying cblique to the beam, 
then the photographic plate would cut the reflected cylindre just in 
an ellipse, whose longest axis is perpendicular to the direction in 
the plane already considered, whereas on the photograms we observe 
just the contrary. 
In the pencil the beams are not wholly parallel. What is the 
influence of this on the diffraction image? If the beams forming a small 
angle will have to give the same reflected beam then the reflecting 
planes must form a small angle too, and otherwise. Now if ov + 
+ by-+cz=0O is the plane rich in molecules, then a plane very 
little differing from it as to its direction will be 
1 1 yi 
(sone tse (cr toae 
Lv ad J 
where p, g, r are large whole numbers; or, 
gr (pa + 1) + (69 + 1 pe + C9 + Dog=9. 
This plane however will be very poor since /, here becomes 
] 
Vr ee 
is thus exclusively ruled by the planes very rich in molecules. Of course, 
each of the pencils in the incident beam gives a reflected pencil to 
a plane rich in molecules, but since the incident beams differ but 
a little, the reflected ones will not do so either. Always, when among 
the planes considered one is rich in molecules the spot will be formed 
by the influence of one of the pencils. 
When we want to consider directly very thin pulses, we come 
to a problem which agrees in some way with the one treated by 
Prof. Lorentz '). However, we can now directly consider the pulses 
reflected by the molecules, which were dealt with in this treatise, 
to be combined to pulses formed by the planes rich in molecules, 
since in this case each of such planes gives only one pulse. This fact 
hinders the coinciding of the pulses considered in the publication mentio- 
ned. Take e.g. pulses originating from a definite set of planes, be the 
which is very small. The forming of the patterns 
b) 
l 
dimension in the direction of the normal /, then we have 7, Pulses, 
m 
é 1) Verslagen Kon.Akad.v.Wet. XX1 1912/13 p.911. „Over den aard der Réntgenstralen”. 
