Similar uncertainties arise with respect to the relative movement of two objects, the eye being supposed to be fixed in space.  When two objects seem to pass one another, it may be that they are both moving in contrary directions, or that one only is moving, or finally, that both are moving in the same direction, the one faster than the other.  Experience and habit here again suggest the interpretation which is most easy, and not unfrequently produce illusion.  Thus, when we watch clouds scudding over the face of the moon, the latter seems moving rather than the former, and the illusion only disappears when we fix the eye on the moon and recognize that it is really stationary.  The probable reason of this is, as Wundt suggests, that experience has made it far easier for us to think of small objects like the moon moving rapidly, than of large masses like the clouds.[36]

The perception of distance, still more than that of direction, is liable to be illusory.  Indeed, the visual recognition of distance, together with that of solidity, has been the great region for the study of “the deceptions of the senses.”  Without treating the subject fully here, I shall try to describe briefly the nature and source of these illusions.[37]

Confining ourselves first of all to near objects, we know that the smaller differences of distance in these cases are, if the eyes are at rest, perceived by means of the dissimilar pictures projected on the two retinas; or if they move, by this means, together with the muscular feelings that accompany different degrees of convergence of the two eyes.  This was demonstrated by the famous experiments of Wheatstone.  Thus, by means of the now familiar stereoscope, he was able to produce a perfect illusion of relief.  The stereoscope may be said to introduce an exceptional state of things into the spectator’s environment.  It imitates, by means of two flat drawings, the dissimilar retinal pictures projected by a single solid receding object, and the lenses through which the eyes look are so constructed as to compel them to converge as though looking on a single object.  And so powerful is the tendency to interpret this impression as one of solidity, that even though we are aware of the presence of the stereoscopic apparatus, we cannot help seeing the two drawings as a single solid object.

In the case of more remote objects, there is no dissimilarity of the retinal pictures or feelings of convergence to assist the eye in determining distance.  Here its judgment, which now becomes more of a process of conscious inference, is determined by a number of circumstances which, through experience and association, have become the signs of differences of depth in space.  Among these are the degree of indistinctness of the impression, the apparent or retinal magnitude (if the object is a familiar one), the relations of linear perspective, as the interruption of the outline of far objects by that of near objects, and so on.