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19th Century Mechanical Models of Eye Movements, Donders' Law, Listing's Law and Helmholtz' Direction Circles

Donders' Law

"De werking en verrigting der oogspieren behoort tot die onderwerpen, welke door physiologen ten allen tijde met ijver en een zekere voorliefde zijn behandeld, en deze ijver vond in de voor eenige jaren zooveel gerucht makende operatiën tegen het scheelzien en de bijziendheid overvloedig voedsel." (The action and effect of eye muscles is one of the subjects that have always been treated by physiologists with zeal and predilection, and this zeal has been nourished by the operations for squint and for myopia that caused so much uproar some years ago.) This first sentence of F.C. Donders' first article on eye muscle mechanics applied in 1846 as it applies today. Donders, Ruete, von Graefe, von Helmholtz, Listing, Volkmann and many others have provided the broad outline of an answer to the question how the eye rotates during eye movements. Donders had become interested in eye movements when he translated Professor C.G.T. Ruete's 'Lehrbuch der Ophthalmologie' (1846). He was a young doctor in the military in Utrecht, earned 800 guilders a year and, hence, did translation work to earn extra money. Ruete (1845) had developed in Scharmbeck the first mechanical model of the eye and its muscles. He called it an 'ophthalmotrope'. Ruete also studied the rotation of his own eye about the visual axis. He was able to observe the rotation of his own eye about the visual axis by using an afterimage with the form of a + cross. A green afterimage was produced by looking at a red cross for a long time. He then looked at a screen in front of him to see whether the afterimage remained vertical when he looked right, left, up or down. Donders repeated these experiments and found that the afterimage cross tilted, when he looked in tertiary positions of gaze (i.e. right-up, right-down, left-up or left-down): He found that the amount of torsion depended upon the amount of elevation or depression and right or left gaze (Donders' Law).
In 1848 a German edition of Donders' work appeared which drew von Helmholtz' attention. He was very enthusiastic about Donders' discovery and proposed to call the definition of pseudotorsion 'Donders' law'. The reason for the tilt had not been recognized by Donders, however, it was von Helmholtz that explained the reason for pseudotorsion.