Please explain why 54 While studying atomic spectra Sherlock

Please explain why

54. While studying atomic spectra, Sherlock comes to the conclusion that these spectra can reveal atomic numbers but not mass numbers. Watson, on the other hand, argues that both are revealed by spectra. Who do you think is correct, and why?

Solution

Due to the isotopes of the elements, spectra of many elements can be same. So we can\'t identify the the mass number of the element of the given spectra.

and from Moseley\'s atomic theory atomic number is the base of identify any spectrum.