PROBLEMS IN ASTROMETRY: Some aspects of the Lick proper motion program

A bstract. The Lick program of determination of stellar proper motions with respect to the extragalactic nebulae is dis- cussed. The importance of measuring the stars at three levels of photographic magnitudes, approximately 8th, 12th and 16th, is pointed out. Approximately 25 stars at each magnitude level and S0 extragalactic nebulae are considered adequate for measurement on each 6 X 6 plate. Some criteria are discussed for the selection of the stars to be measured. The proposed minimum interval of epochs is 20 years; a limited number of plates, however, could profitably be repeated with an epoch difference of 10 years.