What role does the thumb loop serve in using a lensatic compass?

The thumb loop in a lensatic compass serves as a retaining device when the compass is closed. This feature is crucial for ensuring that the compass remains securely closed during transport, which helps prevent damage to the delicate components inside. It ensures that the lid does not open unintentionally, protecting the lens and the internal workings of the compass.

When in use, the loop can assist the user in maintaining a steady grip and control of the compass while taking readings or navigating. However, its primary function is to keep the compass securely closed when it is not in use, facilitating safe and convenient storage and transport. This design consideration adds to the overall durability and functionality of the compass in various field conditions, making it an essential tool for navigation.