Periscopes of German submarines almost invisible due to nickel/aluminium covering according to the Dutch magazine Marineblad dated 1917-1918 no. 6

An item referred to the Moniteur de la Flotte dated 10th November reporting that some masters of French merchant ships stated that the periscopes of German submarines were covered with nickel or aluminium which reflected the colours of the water resulting in an invisibility of the periscopes.