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Why the nonrelativistic potential model and the ultrarelativistic bag model give the same spectra.

It is remarked that the Schroedinger equation with a linear potential and the zero-mass Klein-Gordon equation with a quadratic potential transform to each other by the duality transformation: p..-->..br;r..-->..(1/b)p. Therefore the eigenvalues of these equations are the same. The first equation describes the main features of potential models while it is shown that the second describes the main feature of the spectra of the MIT bag model. This is the reason that one obtains the same spectra from the two physically very different models. The wave functions of these models are connected by a Fourier transform.