She has been to an optician at least twice over the last few months. Holmes, Dr. Watson, and Hopkins head to Yoxley the next morning, and Holmes carefully examines the site. In the study, he notices a recent scratch on the bureau and reasons the murderer intended to access its contents. Smith was killed as his presence interfered with the burglary. No one saw the murderer leave, nor did anyone hear a door opening. Holmes notes that both the corridors, the one leading from the back door and the one leading to the professor's bedroom, are about the same length and lined with coconut matting. Holmes interviews the professor in his bedroom, smoking many Egyptian cigarettes and dropping the ashes all over the floor. The professor claims utter ignorance regarding the murder and ventures that Smith's death may be suicide. Holmes asks about the locked cupboard in the bureau. The professor hands over the key. Holmes inspects the key and returns it immediately, leaving the bureau sealed. Watson asks Holmes if he has a clue, and Holmes tells him the cigarette ashes might reveal the truth. Holmes meets the housekeeper in the garden and has a seemingly unimportant chat about the professor's eating habits; apparently, he has eaten heavily today. Come the afternoon, the three men return to the professor in his room, and Holmes deliberately knocks the cigarettes over as an excuse to get a closer look at the floor. Holmes' suspicion is confirmed – there are footprints in the ash. At that moment, the murderer, appearing exactly as Holmes deduced, emerges from a hiding place in a bookcase. The business unfolded thus: The woman came in secret to the professor's house to obtain some documents, using a duplicate key she gained from one of the former secretaries. She was surprised by Smith, whom she attacked with the nearest object to hand, the sealing-wax knife; she had not intended to kill him. She lost her glasses in the scramble to escape; unable to see clearly, she turned along the wrong corridor and wound up in the professor's room. Although surprised, he hid her. The murderer is, in fact, the professor's estranged wife, Anna, and they are both Russian. Years prior, the pair had been involved with Nihilists; she and a non-violent nihilist friend were both betrayed by the professor for gain. Having finished her jail sentence in Siberia, Anna