nearly midnight
`` If you ever have any idea , Walter , of going into the burglary business , it would be well to ascertain if the safes have any of these little selenium cells as suggested by my friend , Mr. Hammer , the inventor . For by them an alarm can be given miles away the moment an intruder 's bull 's - eye falls on a hidden cell sensitive to light . '' While I was delegated to take Miss Wallace home , Kennedy made arrangements with a small shopkeeper on the ground floor of a building that backed up on the court for the use of his back room that night , and had already set up a bell actuated by a system of relays which the weak current from the selenium cells could operate . It was not until nearly midnight that he was ready to leave the laboratory again , where he had been busily engaged in studying the tortoiseshell comb which Miss Wallace in her weakness had forgotten . The little shopkeeper let us in sleepily and Kennedy deposited a large round package on a chair in the back of the shop , as well as a long piece of rubber tubing . Nothing had happened so far . As we waited the shopkeeper , now wide awake and not at all unconvinced that we were bent on some criminal operation , hung around .