has no means of estimating the distance of C. All that it can tell is, that it is in the direction of the
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Fig. 38.
line AC; but there is no phenomenon of vision by which it can judge accurately of its distance in that line AC. Suppose, now, the other eye B is turned to C, then there is a circumstance introduced which is affected by the distance, namely, the difference of direction of the two eyes. While the object is at C, the two eyes are turned very little inwards to see it; but if the object is brought very close, as for instance to D, then the two eyes have to be turned considerably inwards to see it; and from that effort of turning the eye, we acquire some notion of the distance. We cannot lay down any accurate rule for the estimation of the distance; but we see clearly enough in this explanation, and we feel distinctly enough when we make the experiment, that the estimation of distance does depend upon this difference of direction of the eyes. When the object is brought very near, the feeling becomes very annoying. This is the principle upon which is founded this experiment of which I have spoken. Now, this difference of direction of the two eyes, is a veritable parallax; and this
