Bohdan Paczynski, the Lyman Spitzer Jr. Professor of Theoretical Astrophysics, died April 19 of brain cancer. He was 67. Paczynski, who came to Princeton in 1982, was a pioneer in discovering objects in space and calculating their mass through a technique known as gravitational lensing, in which the gravity of a massive body bends light that passes by. Among the discoveries produced by this technique was the first terrestrial planet found outside our solar system. His research also focused on gamma-ray bursts and the structure of galaxies.