The wavelength dependence of polarization. 1. Instrumental polarization

Measures on n onpolarized stars are reported for the McDonald 82-inch and 36-inch and for the Goethe Link 36-inch reflectors. For different telescopes the amount of instrumental polarization differs greatly. The polarization increases toward the ultraviolet. The instrumental effects may cause photometric as well as polarimetric errors, and a method to avoid them (Thiessen and Broglia 1959) is referred to. Measures on depolarizatio n effects are reported and small corrections are derived for incomplete action of the depolarizer disks and of the Wollaston prism. Depolarization by the mirrors of the 82-inch telescope is found to be negligible.