Far infrared magneto-transmission spectroscopy has been used to probe relativistic fermions in highly oriented pyrolytic and natural graphite. Nearly identical transmission spectra of those two materials are obtained, giving the signature of Dirac fermions via absorption lines with an energy that scales as sqrt{B}. The Fermi velocity is evaluated to be c*=1.02x10^6 m/s and the pseudogap at the H point is estimated to be below 10 meV.
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