Doyle, Arthur Conan - Three mysteries of Sherlock Holmes. The golden pince-nez

One night, Mr. Hopkins went to Holmes’ study where there was Watson too.
He asked them to help him solve a mystery: a secretary of Professor Coram, Mr. Smith, was found murdered in the professor’s study this morning, at Yoxley Old Place, a country house where lives the Professor.
A servant (Miss Tartlon), found the secretary lying on the floor with a wound on the back of his neck. Next to him there was a knife and he had a pair of lady’s glasses in his hand.
The two detectives, went to Yoxley Old Place and Holmes examined the cupboard’s door and he noticed a scratch to the right of the key-hole. Probably, the woman was trying to open it but Mr. Smith saw her.
Then, the investigators spoke to the professor.
Later, a maid, told Holmes that the professor had a very big breakfast; so Holmes understood that he gave some of it to another person.
Before, though, Holmes dropped some ash on the bedroom’s floor: he wanted to see the traces of the door on the ash.
Suddenly, the door moved and a woman came out. She confessed she had killed Mr. Smith, but it was an accident. She said she was the professor’s wife: they got married in Russia, but the professor escaped to England. She and her friend were arrested and she went there to steal some documents that can free her friend: they were in the cupboard and she got the keys from the secretary before Mr. Smith.
The women gave Holmes these documents and asked him to give them to the Russian Embassy. Then, she poisoned herself.