Most melting points you see in papers nowadays are uncorrected. A corrected melting point can be obtained by using a reference substance with a known melting point. If one measures the melting point of this reference compound on any apparatus available (remember the melting point is known, let it be y) one will obtain a certain melting point x. If x=y no correction has to be done. But if it differs, all melting points of samples with unknown melting points have to be corrected by the factor z (difference between x and y).