Exactly corresponding to these relations in the sense of touch are the phenomena that we observe in the formation of visual spatial ideas. Here as well we notice two sensation-components regularly working together. The one consists of the sensations of the retina. Analogous to the touch sensations of the skin, they vary in quality not only according to the constitution of the outer impressions, but also according to the part of the retina which is affected by the impression. The other component consists of the extremely delicate sensations which accompany the positions and movements of the eye. They vary in their intensity according to the length of the distance through which the movement travels, just like the sensations of movement of the other muscles of the body. We notice, therefore, that changes in the position of the retinal elements, which may occur in inflammations of the inside of the eye, or abnormalities in the mechanism of the eye-movements may disturb considerably our spatial perception. They cause sometimes apparent dislocations in the objects seen, and at other times illusions as to their size and distance. These influences can be demonstrated on the normal eye by means of experiments. By making the movement of the eye more difficult, we cause the length of a distance to be over-valued. If we compare two straight lines of exactly the same length, one of which is interrupted by a number of transverse lines, so that a continuous movement of the eye is hindered, then this divided line appears longer than the undivided one. We can also by systematic experiments change the normal relation between eye-movements and retinal sensations. It will then be observed that our vision slowly begins to adapt itself to this new relation between the eye-movements and the position of the retinal elements. This can be done by wearing spectacles with prismatic glasses for a considerable length of time. At first all objects appear distorted. A straight line appears curved, a circle looks like an oval, and so on. If the spectacles are worn for several days, these distortions disappear. It may happen that distortions again appear when the glasses are discarded. This phenomenon can scarcely be accounted for except in the following manner. The retinal sensations by means of local differences in quality, which we may call qualitative local signs, correspond to definite sensations of movement graduated as to intensity, which we may call intensive local signs. Their relation to the centre of the retina probably determines this correspondence. Now our experiment with the prismatic glasses shows that this relation is neither an absolutely permanent nor an innate one, but that it is acquired by practice. It is acquired by the function itself, and therefore, when the functional relations are changed, gives way to a different relation or correspondence. This combination possesses distinctly the character