 Not knowing when his next billed episode would be , Nimoy said in an interview before " Momentum Deferred " aired that he 's " waiting to see what these terribly imaginative writers come up with for the future . I ’ m expecting that I probably will be going back to work for them before too much longer . I 'm looking forward to what they send me on the page " . After shooting the episode , Pinkner said " We want him back as much as he wants to come back " . Nimoy 's next appearance occurred six episodes later , in " Grey Matters " . As a reference to a brief appearance by Nimoy 's character William Bell , sound editor Bruce Tanis was instructed to create bells as a recurring motif in the scene when Bell encounters Olivia and gives her vital information about the parallel universe . Tanis explained that because he couldn 't put the sound of bells in everything , he " took some dry ice recordings and pitched them down considerably and reverbed them to become ringing tones that I played underneath the whole scene " . Tanis also used some " reversed " ship 's bells as well as crystal glass rubs in order to make Bell 's office sound like multiple layers of glass and bell sounds ; he referred to the scene as " a little ominous " but not " too cliche " . 
