One of the first post-focus instruments of the new solar telescope GREGOR will be a Fabry-Perot spectrometer, which is an upgrade of the G\"ottingen Fabry-Perot interferometer at the Vacuum Tower Telescope (VTT) on Tenerife. This spectrometer is equipped with a full-Stokes polarimeter. The modulation is performed with two
ferro-electric liquid crystals, one acting nominally as
quarter-wave plate, and the one as half-wave plate. A modified Savart plate serves as polarimetric beam splitter. With the present liquid crystals, this polarimeter can be used in the wavelength range between 580 and 660\,nm. The optical design and the basic operating procedure will be described. We will show examples of observations obtained with the VTT to illustrate the capabilities of the new telescope GREGOR which will provide a spatial resolution of about 0$^{\prime\prime}$\hspace{-0.15cm}.\,1 (75\,km). Thus we will be able to investigate small magnetic features,
and we will study their development with high cadence.