What is "ophthalmotrope"

An ophthalmotrope (from the Greek roots ὀφθαλμοῦ, ophthalmos, i.e. eye and -τροπή, -tropē, i.e. turning) is an apparatus for demonstrating the movements of the eye and the action of the different muscles which produce them, consisting essentially of a model eyeball to which are attached strings and pulleys to duplicate the line force of the muscles. Movements of the eye are kinematically complex and can be described as a combination of rotations about changing rotation centers. But even when ocular mechanics are simplified to pure rotations about a head-fixed rotation center, their noncommutative property makes them difficult to visualize. Ruete, Listing, Donders, Helmholtz, von Graefe, Volkmann and many others have provided the broad outline of an answer to the question how the eye rotates during eye movements.