If an A. P. L. man knew the chemistry of any synthetic or invisible ink, he would not make the secret public any more than would M. I. D. Many devices for making and using these inks, however, are very generally known, although it is believed that Great Britain and France have gone farthest in classifying and developing them. A piece of a necktie has been taken from one German, a corner of which, snipped off and put in a glass of water, would make an invisible ink. A shoestring has been known to do the same thing, a small piece of it making enough for a letter or more. A shirt-stud has been described by a foreign operative, which, when unscrewed and dropped into a glass of water, would do the same thing and leave no trace. With what chemicals were these articles treated in order to make the ink? Ah, that is another matter. If the author knew, he could not tell. One thing is sure, it is not likely that the most inventive writer of “detective” stories could imagine anything more ingenious or more baffling than some of these well-known methods in use by our own men. Mr. Byron R. Newton, collector at the port of New York, gave out a curious story on the work done by the Customs Intelligence Bureau, created as a lookout for smugglers and others. This service was employed in searching ships, examining baggage, looking out for explosive bombs, invisible writing, and so forth. Mr. Newton’s story appeared in the New York _Herald_ of July 14, 1918, and from it one incident may be taken. Through the Boarding Officials, a passenger who arrived the other day has furnished interesting material for the Intelligence Bureau investigators. The passenger, who for some time had been a resident of Germany, although an American citizen, said he had been approached in Dresden by German agents and asked if on his return to the United States he would obtain military and other information of interest to the Imperial German Intelligence Bureau. He was furnished with a code to be used by him for forwarding information to Germany and also with a formula for manufacturing an _invisible ink_, and with paper to be treated by a special process for correspondence. The passenger, in evidence of what he stated, offered four collars to the customs officials. They appeared to be ordinary negligee collars of cream-colored material—double, turn-over collars, medium height, such as many men wear with sport shirts or for informal occasions. The passenger explained the purpose of these collars as follows: