Existing comment:
Disks and Volvelles:
In the final section of his short text Of Ciphers (the West's first surviving treatise on the subject), Leon Battista Alberti describes the construction and use of his cipher disk: "I make two similar circles, one larger called stable and the other smaller called mobile. I divide the circumference of each into 24 equal parts. Now I write out the alphabet in capitals in the calls of the larger circle (leaving out H, J, K, U, W, and Y) and the numbers 1, 2, 3, and 4 in the remaining four cells. Then a completely random alphabet is written out in small letters on the smaller inner circle. We place a pin through the centers of both circles and fix them on a common axis, around which the inner mobile circle turns. Now in order to use this device, you must have a copy of it, and your friend must also have one with the letters on both circles simlarly arranged an in no wise different. We decide between ourselves the arrangement which we intend as an indicator [i.e. the agreed position of the inner disk]; for an indicator is just like a key which lets us into the secret inner sanctum." |