You take the servants' staircase.

Then it's right at the end of the passage on the left.

Would you like me to take you up to it, sir ?" said the policeman eagerly.

His heart was in his work.
"Thank you, I know where it is--I've just come from it," said Guerchard gently.
A grin of admiration widened the already wide mouth of the policeman, and showed a row of very white, able-looking teeth.
"Ah, M.Guerchard!" he said, "you're cleverer than all the examining magistrates in Paris put together!" "You ought not to say that, my good fellow.

I can't prevent you thinking it, of course; but you ought not to say it," said Guerchard with husky gentleness; and the faintest smile played round the corners of his mouth.
He walked slowly to the window, and the policeman walked with him.
"Have you noticed this, sir ?" said the policeman, taking hold of the top of the ladder with a powerful hand.