A photoelectric parallax scanner designed and constructed at the Yerkes Observatory to measure parallaxes and proper motions at the telescope is described. The concept of the parallax scanner is similar to Griffin's radial velocity spectrometer except that the parallax star and the field stars are monitored separately, thus making the technique purely differential. Observations with the 40-in. refractor under less than ideal conditions gave the error of a single measurement as ±4.5 μ (s.e.). Observations under clear skies and with improved drive and guidance should decrease the error by a factor of 2 to 3.