To another subsection of M I 10 was delegated the censorship of mail to and from the prisoners of war in the various internment camps in the United States. As there were nearly 6,000 of these interned enemies, and as they were permitted, by the regulations, to send nearly 40,000 letters and post-cards a month, no limit being placed on the amount of mail they could receive, the task of censoring this mass of correspondence, most of it in languages other than English, was very far from being a sinecure. The primary object of this censorship was to prevent the passing of objectionable communications, such as attacks on the government or information which might be of value to the enemy. The censors were also constantly on the watch to prevent the prisoners from acting as correspondence intermediaries; that is, from transmitting messages from Germany to German sympathizers in the United States, or _vice versa_. The kind of paper to be used by the prisoners for their correspondence was selected by the subsection with a view to making difficult, if not impossible, the use of secret inks. Thousands of letters, both to and from the prisoners, were submitted to chemical tests for invisible writing, and hundreds of others, which aroused suspicion because of their peculiar wording or unusual marking, were examined for possible messages in code. One prisoner endeavored to communicate with his wife by writing in lemon-juice under the flap of the envelope, and at Fort Douglas a scheme was discovered whereby German sympathizers communicated with the prisoners by means of dots placed under the letters of words in newspapers sent into the internment camp.