In the Brothers Grimm tale “The Twelve Dancing Princesses,” what does the soldier use to become invisible?

In the Brothers Grimm tale “The Twelve Dancing Princesses,” the soldier uses a cloak of invisibility to follow the princesses unseen.

The king’s twelve daughters mysteriously wear out their shoes every night, despite being locked in their room. A soldier accepts the challenge of discovering where they go. An old woman warns him not to drink the wine offered by the princesses and gives him the magical cloak. Wearing it, he secretly follows them through a trapdoor beneath their bedroom.

The princesses travel underground to three spectacular castles, made respectively of copper, silver, and gold, where they dance with twelve princes. The soldier gathers evidence by taking branches from the enchanted trees and drinking cups from the underground palaces.

Retellings sometimes replace the cloak with another invisibility device, but the Grimms’ version specifically uses a cloak. His successful report earns him the right to choose one of the princesses as his wife.