In normal binocular vision the faculties of accommodation and convergence operate in unison. When attention is directed from a relatively distant object to a nearer point, convergence of the eyes is stimulated so that correspondence of the retinal images is maintained. At the same time the need to refocus the retinal images provides a stimulus to accommodation. Thus the basic stimuli to the binocular adjustment for near vision are (a) the temporal displacement of the retinal images relative to one another, (b) the change in the vergence ofthe light reaching the retina. (The term 'vergence' is used throughout to refer to the degree of divergence or convergence of the light: it is measured in dioptres.) Donders (1864) showed that some latitude of action is possible between accommodation and convergence; there is an amplitude of accommodation corresponding to each convergence value, while on the other hand convergence can vary relative to a fixed condition of accommodation. In determining such amplitudes each function in turn requires to be held in a fixed state of action, while the other is varied. The need to provide a stimulus, in order to fix either one of the two mechanisms while the other is changed, leads to the question how far either function, being unstimulated, responds in sympathy to stimulated changes in the other.As it is necessary to avoid the direct stimulation of the mechanism when its induced reaction is being studied, the response of convergence to accommodation can be detected only during monocular fixation, since the need for binocular fusion is the stimulus to convergence. This relationship, accommodative convergence, is well known, and is commonly measured in clinical practice, but there has always been a question whether a similar relation holds in the opposite direction, i.e. the reaction of accommodation to convergence. Thus Christoferson & Ogle (1956) wrote: 'To what extent convergence-accommodation exists is uncertain because the difficulties of measurement have prevented

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