Beta Naphthol: Rogério de Menezes secret writing




The secret ink that was given by the German controller agent in Lisboa to Rogério de Menezes, the Axis agent who worked in the Portuguese Embassy in London, being Armindo Monteiro the Ambassador, was "Beta Naphthol".
With it he sent messages to the Sicherheitdiesnt contact in Portugal and to the Italian informant. 
Looking further details about it I discovered this topic: "dermatitis due to invisible ink". Se it here. The product is exactly the same. 


Once developed, by Mi5 scientist Dr. Henry Vincent Aird Briscoe, using ammonia gas and ultraviolet light to make it fluoresce, the messages could be read for some seconds and photographed.
In the process some of the letters were destroyed. Menezes notice that the letters he was sending to his sister, where the others clandestine letters where hidden, did not arrive.
Strange fact, because they were not sent by post but in the Portuguese diplomatic bag. 
Reason: the British Mi5 was violating it...