It is an old story in physics that higher resolving power leads to new effects. We remember that the magnetic moment of the nucleus was itself discovered through the hyperfine structure of lines in the visible spectrum. The nuclear resonance line in a liquid or gas can be remarkably narrow, as you have already seen. As soon as the reason for this was recognized, it became clear that the only practical limit on resolution was the inhomogeneity of the magnetic field applied to the specimen.

Nobel lecture (11 December 1952)