One of the main limitations, at present, to the accuracy of measurements of long baselines by optical methods is the uncertainty in the average index of refraction over the optical path due to nonuniformity and turbulence of the atmosphere [Thompson et al., 1960]. This uncertainty is due primarily to lack of knowledge of the air density, although a small correction for water vapor is also required. Our purpose in this note is to point out that simultaneous measurements of the optical distance over the same path for blue and red light can be used to provide the required correction for the atmospheric index of refraction. The application of this method to interferometric distance measurements has previously been discussed [Erickson, 1962a].