Broca's pupillometer

pupillometer, Broca's 

A subjective instrument for measuring the diameter of the pupil. It consists of a pair of very small light sources placed at the anterior focal plane of the eye. The two sources are separated until the circular, out of focus images on the retina are seen in juxtaposition. The distance between the two sources represents the diameter of the pupil.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann