The oculus is a tubular device, somewhat conical in shape, with two symmetrically opposed wings, made of wooden staves and fastened with iron hoops. 
It serves to observe the bottom of the sea. At the top, it has an opening, where the observer fits the face from the forehead to the nose. It has a thick polished glass, embedded in the base, well caulked, to prevent the entry of water.
The Collections of the Madeira Ethnographic Museum
Extractive activities: Fishing.

Credits: Madeira Ethnographic Museum