What tool did Edwin Hubble use to identify distant galaxies?

Um, a spectroscope, actually. This is attached to a telescope and provides the spectrogram of the target galaxy. This is basically a long colorful spectrum, with tiny black lines showing up across it. These lines indicate hot ges. The further away an object (i.e. galaxy) is the further toward the red part of the spectrum these lines will lie. This is called "red shift" and can be used to calculate how far away the object is.