What is the purpose of the translucent tube and dial on JU 52 engine nacelle?
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').