"What's that? Number 2

Keratometer - used to measure the shape of the front of the eye (short answer)...or (long answer) it measures the curvature of the cornea (the clear window on the front of the eye). Used in contact lens fitting as well as monitoring conditions that effect the shape of the cornea such as kerotoconus. This particular type of keratometer is known as a Javal-Schiotz. The two coloured lights either side of the observation system, reflect off the cornea. The instrument is then adjusted until the two images just touch and the line through the middle is continuous. The corneal curvature is then displayed in mm of radius. Link here to the full Facebook post.