The spectral series of hydrogen are classified based on the principal quantum number of the final energy state ($n_1$) to which the electron transitions. Each series corresponds to a specific region of the electromagnetic spectrum:

1. Lyman Series ($n_1 = 1$): Transitions to the first energy level fall in the Ultraviolet (UV) region.
2. Balmer Series ($n_1 = 2$): Transitions to the second energy level fall in the Visible region.
3. Paschen Series ($n_1 = 3$): Transitions to the third energy level fall in the Infrared (IR) region.
4. Brackett Series ($n_1 = 4$): Transitions to the fourth energy level fall in the Infrared (IR) region.

Therefore, the Balmer series is the distinct series that produces lines visible to the human eye.