Usually the sender of such correspondence agrees with the recipient as to the way in which the communications shall be read. For example, in 1650 Prince Conde, who lay in prison on account of participation in the plot against the Guises and Catherine de Medici, received a letter which, perused in the ordinary way, could arouse no suspicion. But, in reading it, Conde missed out every other line, obtaining by this means a perfect meaning to fit the situation in which he was placed. There is also a mechanical method of conducting a secret correspondence, for the deciphering of which both sender and recipient must have "castings," or small metal plates exactly similar, which are divided by lines into squares. Some of the squares are cut out, the two plates being kept exactly similar and then the sender lays the square plate on the paper and writes his message in his usual writing on the cut-out squares; then he takes the plate from the paper and fills in the empty squares with casually chosen words, taking care, however, that some meaning is made from the double writing, so as to replace the real meaning when the plate is applied. On receipt of the letter the addressee lays his square on it, and comfortably reads everything that interests him, since the superfluous words which the sender wrote later are covered by the squares in the plate which have been left by the cutting-out. All complex codes and similar means of communication occupy much time, so they are not for spies to use in time of war, though one can employ them in peace time. Certain solutions can be employed as inks, so that, when dry, they leave no trace on the paper; then one has only to warm the paper over a lamp, and the writing stands out clearly. This method, however, has been very largely used, and a blank sheet of paper is always an object of suspicion. And, in view of the meaning which an apparently simple letter may bear to eyes that can decipher the hidden meaning, it should be a rule in war to destroy all captured correspondence of the enemy. This rule should also be observed on the arrest of any suspected person.