midnight
Serve me , my dear Lanyon , and save `` Your friend , '' _ P.S. _ -- I had already sealed this up when a fresh terror struck upon my soul . It is possible that the post office may fail me , and this letter not come into your hands until to-morrow morning . In that case , dear Lanyon , do my errand when it shall be most convenient for you in the course of the day ; and once more expect my messenger at midnight . It may then already be too late ; and if that night passes without event , you will know that you have seen the last of Henry Jekyll . '' Upon the reading of this letter I made sure my colleague was insane ; but till that was proved beyond the possibility of doubt , I felt bound to do as he requested . The less I understood of this farrago , the less I was in a position to judge of its importance ; and an appeal so worded could not be set aside without a grave responsibility .