First, the<unk>  is carefully secured and the penetration needle is brought in contact with the skin. The<unk>  will then give the location of the incision and the depth and direction of the projectile. The<unk>  is then removed and an incision is made in the indicated direction. The surgeons left index finger is put into the wound and the<unk>  is secured in place above the surgeon's left hand. The indicator needle is then pressed into the route prepared by the left index. At the point where the indicator needle ends, the surgeon should be able to feel the bullet which can then be removed. Difficulties and problems with the procedure The procedure requires a large area of skin to be left uncovered, which is sometimes an inconvenience for the surgeon. The main problem with the device is the fact that the patient's soft body parts can change shape and thus give inaccurate results. When the projectile is to be extracted from muscular tissue, the procedure must be done with completely relaxed muscles, therefore needing complete anesthesia. Otherwise, the contractions would displace the foreign object making removal with the aid of the<unk>  difficult.
Hirtz compass
Ocular tilt reaction
Haller index
Cup-to-disc ratio
Krogh length
Stenger test
Mentzer index
Acoustic reflex