This questions has a fascinating answer!

The outer ice giant planets, Uranus and Neptune, had been discovered by telescopes by the mid 1800's. Uranus was looked for and found because of observered perturbations to Neptune's orbit. But perturbations to Uranus' orbit, observed by telescopes in the late 1800's, suggested the presence of ANOTHER body (other than Neptune) whose gravity was also effecting Uranus' orbit. A search began in 1906 but Pluto was not found conclusively until 1930.

Pluto was found by looking at images of the sky several days apart. Objects that are closer to us than the distant stars will appear to move relative to the background star field. Pluto was discovered in this way by comparing images of the skiy taken in January 1930.

Paulett