Astronomical refraction at millimeter wavelengths

Measurements of the difference between radio and optical astronomical refraction at
\lambda=8.6, 4.3, 3.1
, and 2.2 mm are described. The measurement technique utilized solar limb crossing times observed with a 16-foot radio telescope. Results show the expected dependence upon atmospheric water vapor below
\lambda= 3
mm, but radio refraction tends to approach optical refraction at the shortest wavelength.