Albrecht Nagel on vision with two eyes

Nagel's book on vision with two eyes was published in 1861, during a period in which German visual scientists were struggling to rescue the doctrine of identical retinal points from the evidence of stereoscopic depth. The long observational history of binocular vision has been dominated by the appearance of a single world with two eyes and its breakdown when the eyes are distorted abnormally. Early in the nineteenth century the flat horopter of Aguilonius (proposed two centuries earlier) assumed curvature in the form of the Vieth-Müller circle which was linked to identical retinal points: there were only two possible states of binocular perception - singleness with images on the Vieth-Müller circle and doubleness otherwise. This elegant edifice was undermined when Wheatstone demonstrated singleness and depth from images with slight retinal disparities. Nagel responded by providing observations on combining simple line stimuli in the two eyes. In the last part of chapter 3 of his book, Nagel describes experiments with lines varying in orientation or curvature with respect to the two eyes; it is in this section that Nagel draws attention to cyclofusion and the involvement of the extraocular muscles in it. Ocular torsion was an issue of considerable contention in nineteenth century visual science. The possibility of torsion in opposite directions seemed fanciful and yet this is what Nagel proposed in order to maintain cyclofusion for lines inclined in opposite directions relative to the horizontal. Similar rotations about the vertical resulted in a depth effect with no cyclovergence. The involvement of cyclovergence remained hotly debated until photographic recording of eye movements verified it.