illustration from Das Scach Order
by August, Duke of Brunswick-Luneburg
It is certainly true that Pluto's helmet had this faculty of invisibility. Bacon himself in his private notebook the Promus (see Chapter 33) made an entry (Folio 97, reverse No. 705): "Pluto's Helmet; secrecy, invisibility". Pluto was the Roman name for Hades, and, according to a Baconian, it was from Hades that Pallas borrowed her helmet (and used it to cover the foot soldiers of 100 towns). --Nigel Cockburn from The Bacon-Shakespeare Question
from Peter Dawkins' excellent book Dedication to the Light
In this engraving from a good friend and colleague of Francis Bacon's, the dinner of a cabbalistic 'lodge' is depicted, similar to that described by Francis Bacon in his book The French Academie (published anonymously). The group of nine consist of "the tutor"(standing, hatted, back to fireplace), the "four fathers" (seated, and hatted), and the "four sons" (without hats---three seated and one pouring the wine). The spiritual presence of the "wise old gentleman", whose house it is, is understood and represented by the fire, giving the perfect cabbalistic 'Ten', or the 'Nine from One.' They meet and dine on the chequered 'Mosaic floor', in true fellowship and "rememberance". The picture repays study, for it is filled with meaningful symbol, from the numerology of various parts to the design of the furniture, clothing, postures adopted, gestures made, and so forth. The hats here are as used by Francis Bacon in his "Order of the Knights of the Helmet"--the hat, like the helmet of Pallas Athena, being a symbol of wisdom-knowledge (i.e. illumination) and invisibility (i.e. to the ignorant, who would not understand or 'see', and also indicating secrecy--the protection and preservation of that which is sacred from being profane or abused by the wilful ignorant). The Duke of Brunswick-Luneburg was responsible for having the plates engraved for Gustavi Seleni's book, Crypotemenytices et Cryptographiae, a book on cipher in which it is believed for several good reasons that Bacon played a leading role in compiling and/or even writing.
SirBacon.org - Sir Francis Bacon's New Advancement of Learning