Determining optical depth in different parts of the Fraunhofer line contour

The method of approximate determination of the optical depth tau in the profile of the Fraunhofer line is considered. In the method the observed profile and allocation of tau to the temperature in the photosphere must be known. If the source function B prime (tau) in the line seems not to be equal to the Planck function, the dependence B prime (tau) on tau is needed. The method for the determination of B prime (tau) using the center-to-limb observations of the central intensities of the line is suggested.