Direct construction of the quantum field operators of the D=2 Ising model

It is shown that one can take the 'square root' of the field operator describing the disorder variable of the double D=2 Ising model (or equivalently the XY model) recently introduced. The n-point vacuum expectation values of this new field agree in the euclidean region with the correlation functions obtained by McCoy, Tracy and Wu in the scaling limit. This 'square root operator' written in terms of Majorana fields is identical to the operator of Sato, Miwa and Jimbo constructed by a 'Clifford rotation'. (Auth.)