There are two parts to this, in Mission 1 (A Long Day in Dunwall) and Mission 9 (Death to the Empress).
In Mission 1, enter Dr. Galvani's house, the first house to the left after escaping Dunwall Tower and reaching the street level. Take care of the guard, steal the painting above the fireplace ("The Obtuse Argument of Lady Boyle") to find the safe combination (451) and rob the safe.
In Mission 9, reenter Dr. Galvani's house (this time the last on the right, closest to the Dunwall Tower Gate). The trophy should pop after you open the safe.