The speed of apperception has been subjected to measurement by Auerbach, Kries, Baxt, von Tigerstedt and Bergqvist, Stern, Vaschide, Vurpass, etc. The results show 0.015 to 0.035 seconds for compounded images. Unfortunately, most of these experiments have brought little unanimity in the results and have not compared, e.g., the apperception-times of very clever people with those of very slow and stupid ones. In the variety of perception lies the power of presentation (in our sense of the term). In the main other forces assist in this, but when we consider how the senses work in combination we must conclude that they determine their own forms. “If we are to say that sense experience instructs us concerning the manifoldness of objects we may do so correctly if we add the scholium that many things could not be mentioned without synthesis.” So Dörner writes. But if we approach the matter from another side, we see how remarkable it is that human perceptions can be compared at all. Hermann Schwarz says “According to the opinion of the physicists we know external events directly by means of the organs, the nerves of which serve passively to support consciousness in the perception of such events. On the contrary, according to the opinion of most physiologists, the nerve fibers are active in the apprehension of external events, they modify it, alter it until it is well nigh unrecognizable, and turn it over to consciousness only after the original process has undergone still another transformation into new forms of mechanical energy in the ganglion cells of the outer brain. This is the difference between the physical theory of perception and the physiological.” In this connection there are several more conditions pertaining to general sense-perception. First of all there is that so-called vicariousness of the senses which substitutes one sense for another, in representation. The _actual_ substitution of one sense by another as that of touch and sight, does not belong to the present discussion. The substitution of sound and sight is only apparent. E. g., when I have several times heard the half-noticed voice of some person without seeing him, I will imagine a definite face and appearance which _are_ pure imagination. So again, if I hear cries for help near some stream, I see more or less clearly the form of a drowning person, etc. It is quite different in touching and seeing; if I touch a ball, a die, a cat, a cloth, etc., with my eyes closed, then I may so clearly see the color of the object before me that I might be really seeing it. But in this case there is a real substitution of greater or lesser degree.