What is the purpose of the translucent tube and dial on JU 52 engine nacelle? Mounted on top of each JU 52 engine is a translucent tube and a dial (see images).


What is the purpose of the tube and what does it measure?
Does the dial measure (oil or fuel) flow or quantity?
Are these indicators visible at night?
This is the Fuel Gauge.

Direkt auf der Triebwerksgondel sind bei der Ju 52/3m die Treibstoffanzeigen montiert
The fuel guage is mounted directly on the engine pylons on the Ju 52/3m.
AustrianWings

It is suggested that the indicator gives a reading in hundreds of liters (Liter x 100 written underneath).

Source
Visible on the illustration below as Kraftstoffuhr ('Fuel Clock').