What instrument is used to measure proptosis or exophthalmos in millimeters?

The instrument specifically designed to measure proptosis or exophthalmos is known as the exophthalmometer. This tool is essential in assessing the degree of bulging of the eye, which can be an important indicator of various ocular conditions, such as Graves' disease or other thyroid-related disorders.

The exophthalmometer typically features a scale that allows the clinician to take precise measurements in millimeters, which helps in diagnosing and monitoring conditions that lead to eye prominence. By positioning the device in front of the patient's eyes, the clinician can gauge the distance the eye protrudes from the orbital rim, providing valuable data for clinical evaluation.

Other instruments listed, such as the ophthalmoscope, are primarily used for examining the interior structures of the eye; retinoscopes are utilized in refraction to determine a patient's prescription for glasses; and tonometry is focused on measuring intraocular pressure. While all these tools are important in ophthalmology, none are designed for the specific purpose of measuring proptosis as accurately as the exophthalmometer.