The viewers look like televisions. There are two of them, one to look into the future, and one to look into the past. They are connected by an intricate set of wires. They are also connected to a paradox sink, which looks like an old-fashion water mine.
X-rays of the viewers show that the components inside them were welded together and to the frame, making it impossible to take them apart without damaging them.
They are housed inside the moment.
The viewers originally had the ability to look into any loop, but were later limited to only the current loop and one loop forward. B3 used them to look into future loops B4 through B13. However, the viewers did not have the ability to see the end of any loop, so B3 did not know what caused them to fail or reset.
The viewers were also used to choose an exit point for the moment. Bina would have to use them to choose where and when she wanted to land when she exited the moment, or else the moment would deposit her at a random point in her timeline. This was later changed when B13 reset the moment and was able to access its controls directly.
The viewers are what B12 used to leave notes, supplies, and the moment for B13.
B3's Sabotage Edit
Once B3 learned that looking into other loops doomed them to fail after seeing a note left from Josephine in B13's loop, she sabotaged them so that other iterations of herself couldn't repeat her mistakes. She built in limiters that would only allow the other Bina's to look inside their own timeline, and one loop forward to help the next iteration. She also made them necessary for exiting the moment so that future Binas wouldn't risk tampering with them. She did, however, build in a work-around that B13 and Kendra used to communicate with her the first time they used the moment.
They no longer work in B13's timeline. After B13 discovered the work-around, which was that an entire wall of the viewer was composed entirely of the limiters B3 built and not necessary for the machine to function, and also contained a switch that would easily allow Bina to by-pass that entire section, she smashed it with a sledgehammer. Kendra then, in a far more safe and self-preserving manner, flipped the switch. When the viewers turned on again, they acted as a kind of video call between B3 and B13's loop. B3 explained why she sabotaged the viewers, and then taught B13 how to reset the moment so that she could access its controls direct from the pocket watch and by-pass having to use the viewers to set an exit point. She then stated that the viewers would not work after the conversation ended.
Kendra says that they are the second most expensive machines entropy-wise that any of the Binas built, after the moment itself. This means that they created a lot of entropy, and that they would not be able to rebuild them, or should not for the sake of keeping the timeline intact.