1. IN AN EVACUATED VACUUM CHAMBER HAVING AN EVACUATING PUMP MEANS AND WHEREIN IT IS DESIRED THAT SUBSTANTIALLY ALL MOLECULAR BOUNCE-BACK OF MOLECULES LIBERATED FROM AN OBJECT BEING TESTED THEREIN BE ELIMINATED, THE IMPROVEMENT COMPRISING: A STRUCTURE FOR FORMING WITHIN SAID CHAMBER A DEFINED AREA FOR LOCATING SAID OBJECT, SAID STRUCTURE PROVIDING SUBSTANTIALLY NO WALL SURFACE PERPEN-