If in Fig. 4 both eyes are fixed upon and see distinctly the point a, a distinct retinal image will be formed in both eyes upon
Fig. 4.
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the macula lutea at m. Now, if we place before the eyes prisms with their refracting angles turned outward, the rays proceeding from a will be refracted towards m', and the point a will be seen double. Binocular single vision will, however, be soon re-established by a lateral turning of the cornea, the macula lutea being moved from m to m'. The deviation caused by the prisms will be balanced by a compensating deviation of the visual axes, and it is easy to determine within what limits this is possible. Conversely, by the use of prisms with their bases turned outward, the visual axes may be caused to intersect at a point lying nearer to the eye than a, while the accommodation remains adjusted for that point.
The results appear even more striking in the method usually employed by Donders, which determines the limits of accommodation, while the convergence of the visual axes remains unchanged.
If, for instance, the object-point a in Fig. 5 is at a distance
Fig. 5.
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of 12 inches from the eyes, it can be determined, while the visual angle remains the same, with what concave and convex
