e been yet solved.

It certainly appears, according to Professors Haga and Wind and to
Professor Sommerfeld, that with the X rays curious experiments of
diffraction may be produced. Dr Barkla has shown also that they can
manifest true polarization. The secondary rays emitted by a metallic
surface when struck by X rays vary, in fact, in intensity when the
position of the plane of incidence round the primary pencil is
changed. Various physicists have endeavoured to measure the speed of
propagation, but it seems more and more probable that it is very
nearly that of light.[27]

[Footnote 27: See especially the experiments of Professor E. Marx
(Vienna), _Annalen der Physik_, vol. xx. (No. 9 of 1906), pp. 677 _et
seq._, which seem conclusive on this point.--ED.]

I must here leave out the description of a crowd of other experiments.
Some very interesting researches by M. Brunhes, M. Broca, M.
Colardeau, M. Villard, in France, and by many others abroad, have
permitted the elucidation of several interesting problems relative to
the duration of the emission or to the best disposition to be adopted
for the production of the rays. The only point which will detain us is
the important question as to the nature of the X rays themselves; the
properties which have just been brought to mind are those which appear
essential and which every theory must reckon with.

The most natural hypothesis would be to consider the rays as
ultra-violet radiations of very short wave-length, or radiations which
are in a manner ultra-ultra-violet. This interpretation can still, at
this present moment, be maintained, and the researches of MM. Buisson,
Righi, Lenard, and Merrit Stewart have even established that rays of
very short wave-lengths produce on metallic conductors, from the point
of view of electrical phenomena, effects quite analogous to those of
the X rays. Another resemblance results also from the experiments by
which M. Perreau established that these rays act on the electric
resistance of selenium. New and valuable arguments have thus added
force to those who incline towards a theory which has the merit of
bringing a new phenomenon within the pale of phenomena previously
known.

Nevertheless the shortest ultra-violet radiations, such as those of M.
Schumann, are still capable of refraction by quartz, and this
difference constitutes, in the minds of many physicists, a serious
enough reason to decide them to reject the more simple hypothes