The experimental position improved dramatically after research by Henry Moseley in 1913. Moseley, after discussions with Bohr who was at the same lab (and who had used Van den Broeks hypothesis in his Bohr model of the atom), decided to test Van den Broeks and Bohrs hypothesis directly, by seeing if spectral lines emitted from excited atoms fitted the Bohr theorys postulation that the frequency of the spectral lines be proportional to the square of Z.
To do this, Moseley measured the wavelengths of the innermost photon transitions (K and L lines) produced by the elements from aluminium (Z = 13) to gold (Z = 79) used as a series of movable anodic targets inside an x-ray tube. The square root of the frequency of these photons  increased from one target to the next in an arithmetic progression. This led to the conclusion (Moseleys law) that the atomic number does closely correspond (with an offset of one unit for K-lines, in Moseleys work) to the calculated electric charge of the nucleus, i.e. the element number Z. Among other things, Moseley demonstrated that the lanthanide series (from lanthanum to lutetium inclusive) must have 15 members—no fewer and no more—which was far from obvious from known chemistry at that time.