The Divergence of the Primary Position of the Eyes.

I HAVE previously pointed out' why the text-book conception of the Primary Position is wrong. It is essential to establish the actual physiological Primary Position, because it is the basis of kinetic binocular ophthalmology. My view is that the anatomical position is the PRIMARY POSITION of the bi-unial eye. This position is slightly divergent. I propose to call this divergence the " PRIMARY DIVERGENCE." The consequences of Primary Divergence will now be considered. The visual axis of the eye connects three points, viz: the fovea, the nodal point of the dioptric system, and the fixation point of the visual object. The intra-ocular part of this line is practically straight. The course of the extra-ocular part is determined by the refracting surfaces it passes through between the cornea and the fixation point. If there are no refracting surfaces, this part is also a straight line. In bi-foveal single vision there are four possibilities, illustrated by the Figs. 1, 2, 3 and 4. Text-books call Fig. 4 the "negative convergence." 256