In this paper, we propose a purely macroscopic theory of the complex refractive index based on dielectric models defined by constitutive equations which are linear differential equations in the electric field, the polarization vector and their time derivatives up to the orders m and n respectively. Meixner's thermodynamics restrictions impose 0 ≤ n — m ≤ 2 and a set of inequalities on the constitutive coefficients. In the special case m = 2, n — 4, the expression of the complex refractive index shows the existence of two optical resonances. In the general case, it seems that the number of resonance frequencies is the greater integer contained in n/2
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