What does Nicodemus take from his satchel to help him see and why does he need it in Mrs frisby and the rats of nimh?

He tells her about how they were captured and taken to NIMH and they were tested on and it seemed they weren't aging or getting sick. They soon became very intelligent. They learned how to read and escape from their cages. They would sneak out of their cages every night and hold meetings. In one of theses meetings they decided to escape by the air vents. The mice in the cabinet heard them and they wanted to go too. The night they escaped through the air vents there was a big wind and a few mice and rats got lost.