For the interpretation of the physical events occurring at the beginning of a flare it seems to be significant not only to observe the spectra of the short-wavelength X-ray radiation but also its polarization. Solar X-ray spectra below 10 Å during flares observed by Bowen et al. (1964) show in the region of long wavelength a slow fall in their intensity and below 5 Å approximately a rapid fall (Figure 1). This spectral variation cannot be understood on the basis of thermal radiation only. However, it is possible to explain the observed spectra by assuming a superposition of a thermal radiation as well as a non-thermal one. Indeed, one should assume that the fast electrons, during their acceleration, obtain a preferred direction and that only afterwards their velocity distribution becomes a Maxwellian one. The assumption of the existence of a non-Maxwellian distribution of electrons in the beginning of a flare is also supported by the fact that the rise in intensity by more than one order of magnitude takes place in a few minutes. This point has already been emphasised by Bowen et al.