It was after many long researches and thinking to repeat the success of Le Verrier finding a planet through the perturbations on another one thazt Pluto was finally discovered on February 18, 1930 by Tombaugh. The planet is magnitude 15 and can be observed apparent disk.
This discovery is, in fact, due to chance, Pluto having a mass too small to disrupt the motion of Neptune significantly.
Left, images of the discovery of Pluto: a small punctual object moving very slowly on a photographic plate. From top image to bottom, Pluto moved upwards.