Abstract: A positional measuring laser interferometer splits a laser beam into a measuring beam and a correcting beam. Each of these beams is further split into a measurement beam and a reference beam. The reference beams travel along a fixed length path of fixed optical properties. Each of the measurement beams travels along respective optical paths in air adjacent each other so that they travel in air having substantially the same changing optical properties, such as the fraction index, preferably with the paths differing in length by a fixed amount regardless of the movement of the object to be measured. The reference beam and measurement beam, for each of the measuring and correcting beams, form an interference pattern that is detected to produce respective outputs correlated to positional information of the object and subjected to positional error due to changing refraction index of air.