187.
Let the sun shine diagonally into an empty cubical vessel, so that the opposite side be illumined, but not the bottom: let water be then poured into this vessel, and the direction of the light will be immediately altered; for a part of the bottom is shone upon. At the point where the light enters the thicker medium it deviates from its rectilinear direction, and appears broken: hence the phenomenon is called the breaking (brechung) or refraction. Thus much of the objective experiment.
188.
We arrive at the subjective fact in the following mode:—Let the eye be substituted for the sun: let the sight be directed in like manner diagonally over one side, so that the opposite inner side be entirely seen, while no part of the bottom is visible. On pouring in water the eye will perceive a part of the bottom; and this takes place without our being aware that we do not see in a straight line; for the bottom appears to us raised, and hence we give the term elevation (hebung) to the subjective phenomenon. Some points, which are particularly remarkable with reference to this, will be adverted to hereafter.
189.
Were we now to express this phenomenon generally, we might here repeat, in conformity with the view lately taken, that the relation of the objects is changed or deranged.
190.
But as it is our intention at present to separate the objective from the subjective appearances, we first express the phenomenon in a subjective form, and say,—a derangement or displacement of the object seen, or to be seen, takes place.
191.