X-rays series of hydrogen is the ordinary Balmer series in the visible region, the head of which is at
In other words, hydrogen’s ordinary radiations are its X-rays and nothing more.
There is also an
series for hydrogen discovered by Paschen in the ultra-red, which in itself would make it probable that there are series for all the elements of longer wave-length than the
series, and that the complicated optical series observed with metallic arcs are parts of these longer wave-length series. As a matter of fact, an
series has been found for a considerable group of the elements of high atomic number.