The Silph Scope is a Key Item obtained in the Generation I games as well as Pokémon FireRed Version and Pokémon LeafGreen Version. The player can use it to view the ghost in the Pokémon Tower in Lavender Town. Without it, the player character cannot get past the ghost Marowak to continue up the tower.
To get the Silph Scope, Giovanni must be defeated in the Rocket Hideout. After all of his Pokémon are defeated, he will disappear and leave it behind.