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ns； （not； therefore； passive as according to Helmholtz； but active functions of the sense organs）。 Perception arises when we combine our particular sensations with the pure images of the spirit or the schemata of the understanding； especially with the pure image of space。 The so…called ejection or externalization of sensations occurs only as their scheme and relation to the unity of their object。''
So long as anything is conceived as passive it may always recur more identically than when it is conceived as active。 In the latter case the individuality of the particular person makes the perception in a still greater degree individual； and makes it almost the creature of him who perceives。 Whether Aubert is right or not is not our task to discover； but if he is right then sense…perception is as various as is humanity。 The variety is still further increased by means of the comprehensive activity which Fischer＇1＇ presupposes。 ‘‘Visual perception has a comprehensive or compounding activity。 We never see any absolute simple and hence do not perceive the elements of things。 We see merely a spatial continuum； and that is possible only through comprehensive activityespecially in the case of movement in which the object of movement and the environment must both be perceived。'' But each individual method of ‘‘comprehension'' is different。 And it is uncertain whether this is purely physical； whether only the memory assists （so that the attention in biased by what has been last perceived）； whether imagination is at work or an especial psychical activity must be presupposed in compounding the larger elements。 The fact is that men may perceived an enormous variety of things with a single glance。 And generally the perceptive power will vary with the skill of the individual。 The narrowest； smallest； most particularizing glance is that of the most foolish； and the broadest； most comprehensive； and comparing glance； that of the most wise。 This is particularly noticeable when the time of observation is short。 The one has perceived little and generally the least important； the other has in the same time seen everything from top to bottom and has distinguished between the important and the unimportant； has observed the former rather longer than the latter； and is able to give a better description of what he has seen。 And then； when two so different descriptions come before us； we wonder at them and say that one of them is untrue。＇1b＇
＇1＇ E。 L。 Fischer： Theorie der Gesichtswahrnehmung。 Mainz 1891。
＇1b＇ Cf。 Archiv； XVI； 371。
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 Drner 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 trans… formation 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。
The same vicariousness occurs when perception is attributed to one sense while it properly belongs to another。 This happens particularly at such times when one has not been present during the event or when the perception was made while only half awake； or a long time ago； and finally； when a group of other impressions have accompanied the event； so that there was not time enough； if I may say so； properly to register the sense impression。 So； e。 g。； some person； especially a close friend； may have been merely heard and later quite convincingly supposed to have been seen。 Sensitive people； who generally have an acuter olfactory sense than others； attach to any perceived odor all the other appropriate phenomena。 The vicariousnesses of visual sensations are the most numerous and the most important。 Anybody who has been pushed or beaten； and has felt the blows； will； if other circumstances permit and the impulse is strong enough； be convinced that he has seen his assaulter and the manner of the assault。 Sometimes people who are shot at will claim to have seen the flight of the ball。 And so again they will have seen in a dark night a comparatively distant wagon； although they have only heard the noise it made and felt the vibration。 It is fortunate that； as a rule； such people try to be just in answering to questions which concern this substitution of one sense…perception for another。 And such questions ought to be urgently put。 That a false testimony can cause significant errors is as obvious as the fact that such substitutions are most frequent with nervous and imaginative persons。
Still more significant is that characteristic phenomenon； to us of considerable importance； which might be called retrospective illumination of perception。 It consists in the appearance of a sense… perception under conditions of some noticeable interruption； when the stimulus does not； as a rule； give rise to that perception。 I cite a simple example in which I first observed this fact。 Since I was a child t