Three lectures, 4, 11 and 18 March, 18:00, Great Hall. Demand has exceeded the hall for the last four years, so tickets are allocated by ballot rather than first come, first served. Enter once, for as many of the three as you want; entering for all three does not reduce your chance of getting any one of them. The ballot closes on 20 February and results go out the same evening. Unclaimed seats are released to a waiting list at 17:45 on the night, which is why we ask ticket holders who cannot come to tell us rather than simply not appearing. Staff and students of the university do not need a ticket for the third lecture, which is part of the teaching programme.