A little practice will enable the observer to realize and to maintain the singular binocular vision which replaces the real picture.[35] The occasional retention of the picture after one eye is closed, and even after both have been closed and quickly reopened, shews the influence of time over the evanescence as well as over the creation of this class of phenomena. On some occasions, a singular effect is produced. When the flowers or figures on the paper are distant _six_ inches, we may either unite two six inches distant, or _two_ twelve inches distant, and so on. In the latter case, when the eyes have been accustomed to survey the suspended picture, I have found that, after shutting or opening them, I neither saw the picture formed by the two flowers _twelve_ inches distant, nor the papered wall itself, but a picture formed by uniting all the flowers _six_ inches distant! The binocular centre (the point to which the optic axes converged, and consequently the locality of the picture) had shifted its place, and instead of advancing to the real wall and seeing it, it advanced exactly as much as to unite the nearest flowers, just as in a ratchet wheel, when the detent stops one tooth at a time; or, to speak more correctly, the binocular centre advanced in order to relieve the eyes from their strain, and when the eyes were opened, it had just reached that point which corresponded with the union of the flowers _six_ inches distant. [35] A sheet of Queen’s heads may be advantageously used to accustom the eyes to the union of similar figures. [Illustration: FIG. 25.]