id 
We may also observe, that in the direction of the propagation of 
the primary radiation too an interference can be noticed between 
the secondary pulses emitted and the primary radiation. At this 
interference a difference of phase shows itself, which to such a degree 
diminishes the primary radiation as is necessary to deliver the energy 
of secondary pulses emitted in the directions of reflection. 
We can still somewhat nearer consider the influence of a single 
plane. Be the reflecting plane chosen as yz-plane, be the xy-plane 
the plane of incidence, and a@ the angle of incidence. Let us now 
consider the vector of radiation in a point 
® == F COS Ot, y=rsina-+ mn, oe 
The vector of radiation is given by 
Pe Ren t r k‚a ka 
— & cos 2m | ——— 4 n + GEN 
ey ay 1 À ar ar 
which, when summed up with respect to &, and #,, will give 
A t u a ar Ld 
nn 7) cos N—! me cos N = ar sin( N + hoe mx sin( NH ne : 
r A 22 2d 2A 2A 
„anr, aga 
Sir Oh sin Oh 
For 7 =0 $¢=0 we obtain the maximum found above (diffraction 
maximum of the order zero) with the intensity there given. 
A second maximum (first maximum of diffraction) could appear if 
ya aS 22 2 
— =1, or —=1, or thus if y= — or 5=—. Now r is about 4 
2) 2A a a 
in the experiments, and « is of the order 10-8; should 2 be much 
smaller than a, then this second maximum would be observable. In 
the photograms we do not find diffraction-rings of this kind. Thus if 
the wavelength is very small with respect to 10~-§ then such images 
do not occur, but if 4 is of the order of a or not much smaller, 
then we can neither observe such images, the latest estimation 
giving for 4 a quantity of the order 10~°. This might well thought 
to be consistent with the result that circular fringes do not appear 
on the plates. 
Brace has explained the form of the spots, — ellipses whose long 
axis has the direction of the line perpendicular to the plane of inei- 
dence which belongs to the plane observed — by observing that the 
different layers are struek by waves not wholly parallel. However, 
he does not take into account that in each point the radiation of 
molecules of all the planes interferes. The form might rather 
be explained by observing that the intensity in the said direction 
