The three-mirror lens is a contact lens for the slit lamp. Through the central part you look at the central retina; the three mirrors at different angles bring the mid-periphery, the far periphery and the anterior chamber angle into view.
This makes the lens suited to gonioscopy, to assessing retinal tears and to following patients with glaucoma. It fits the common slit lamps; use it with a suitable contact medium and clean it as the manufacturer prescribes.
