What discovery occurred after Mendeleev's periodic table was published that allowed Moseley to improve it?

Mendeleev

The next milestone in the development of the periodic table was set by the Russian chemist Dmitri Mendeleev, who is generally acknowledged as the "father" of the modern periodic table. Mendeleev wrote out the names of the elements, along with their atomic weights and other properties, on cards, which he then laid out in rows and columns much like a game of solitaire. When the elements were ordered according to atomic weight, Mendeleev, like de Chancourtois and Newlands, could see that certain chemical properties were repeated periodically; however, not all the elements fit this pattern neatly. Mendeleev's solution was to move certain elements to new positions, despite their accepted weight, in order to group them with other elements sharing similar properties. (Nearly half a century later, after the periodic table was revised according to atomic number rather than atomic weight, these elements fell into place.)

Mendeleev's work on periodic law-which states that the properties of elements recur periodically as their atomic weights increase-was announced in 1869. At about the same time, a German chemist named Julius Lothar Meyer independently arrived at a periodic table that was remarkably similar to Mendeleev's. Unfortunately for Meyer, Mendeleev presented his work to the scientific community first. However, Mendeleev's table was also superior to Meyer's because he left a number of empty spaces to account for elements that were yet to be discovered.