Abstract: An examination instrument for optical apparatus, including a pair of optical heads mounted for relative movement toward and away from one another upon an elongated support, and having means (e.g., an electrical transducer) for producing an electrical signal which is specifically representative of the distance between the optical heads at various positions of relative movement therebetween, for use in analyzing test data obtained by the optical heads. The instrument also includes a test sequence actuation switch disposed in a position where the operator may readily initiate a test sequence without changing his grip upon the instrument as it is held in place on the test subject. The optical heads are mounted on stand-off members which position the heads in spaced relation alongside the elongated support and at an angle relative to one another, to facilitate use on the human breast.