4 0 D O C . 6 I N D I C E S O F R E F R A C T I O N F O R X - R A Y S
then the refraction law takes the form
With negative , total reflection will occur. The limiting angle of total reflec-
tion is determined by ; and therefore
With in the order of magnitude , is of the order of magnitude .
One can well imagine that the bright fringe in Köhler’s pictures is caused by rays
which hit the object almost tangentially, suffer a slight deflection by total reflec-
tion, and thus add their effect to the X-rays that pass close to the object.
The realization is less convenient with a positive , because in this case total
reflection has to occur at a slightly curved, concave part of the surface, toward the
inside of the body.
With our complete ignorance about the refraction of X-rays, it would be very
desirable if a colleague who is experienced in the field of X-ray pictures would
investigate the problem whether or not this is a true case of total reflection.
Note Added in Proof. Also in case of , one should see a phenomenon at
the edges if the picture pertains to rounded objects, because of the refraction of
quasi-tangentially incident rays. Obviously, one has to expect a narrow darker
fringe along the border of the shadow at the inside of the geometric shadow
boundary. determines the width of this fringe.
n 1 ε + =
ε = –
ψ′ 0 =
ψ –ε =
ε) – (