midnight
In my extreme distress of wind , I have a morbid fear of misdirecting you ; but even if I am in error , you may know the right drawer by its contents : some powders , a phial and a paper book . This drawer I beg of you to carry back with you to Cavendish Square exactly as it stands . `` That is the first part of the service : now for the second . You should be back , if you set out at once on the receipt of this , long before midnight ; but I will leave you that amount of margin , not only in the fear of one of those obstacles that can neither be prevented nor fore - seen , but because an hour when your servants are in bed is to be preferred for what will then remain to do . At midnight , then , I have to ask you to be alone in your consulting-room , to admit with your own hand into the house a man who will present himself in my name , and to place in his hands the drawer that you will have brought with you from my cabinet . Then you will have played your part and earned my gratitude completely .