  We have developed and tested an antireflection (AR) coating method for
silicon lenses at cryogenic temperatures and millimeter wavelengths. Our
particular application is a measurement of the cosmic microwave background. The
coating consists of machined pieces of Cirlex glued to the silicon. The
measured reflection from an AR coated flat piece is less than 1.5% at the
design wavelength. The coating has been applied to flats and lenses and has
survived multiple thermal cycles from 300 to 4 K. We present the manufacturing
method, the material properties, the tests performed, and estimates of the loss
that can be achieved in practical lenses.
