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ring some misunderstanding may be popularly assumed。 But the people know little of optical illusions and false visual perceptions； though they are aware that incorrect auditions are frequent matters of fact。 Moreover； to the heard object a large number of more or less certain precautionary judgments are attached。 If anybody； e。 g。； has _*heard_ a shot； stealthy footsteps； crackling flames； we take his experience always to be _*approximate_。 We do not do so when he assures us he has _*seen_ these things or their causes。 Then we take thembarring certain mistakes in observation；to be indubitable perceptions in which misunderstanding is impossible。
In this； again； is the basis for the distrust with which we meet testimony concerning hearsay。 For we feel uncertain in the mere absence of the person whose conversation is reported； since his value can not be determined。 But a part of the mistrust lies in the fact that it is not vision but the perennially half…doubted hearing that is in issue。 Lies are assigned mainly to words； but there are lies which are visual （deceptions； maskings； illusions； etc。）。 Visual lies are； however； a diminishing minority in comparison with the lies that are heard。
The certainty of the correctness of vision lies in its being tested with the sense of touch；i。 e。 in the adaptation of our bodily sense to otherwise existing things。 As Helmholtz says； ‘‘The agreement between our visual perceptions and the external world； rests； at least in the most important matters； on the same ground that all our knowledge of the actual world rests on； upon the experience and the lasting test of their correctness by means of experiments； i。 e。； of the movements of our bodies。'' This would almost make it seem that the supreme judge among the senses is the touch。 But that is not intended； we know well enough to what illusions we are subject if we trust the sense of touch alone。 At the same time we must suppose that the question here is one of the nature of the body； and this can be measured only by something similar； i。e。； by our own physical characteristics； but always under the control of some other sense； especially the sense of sight。
The visual process itself consists； according to Fischer； ‘‘of a compounded series of results which succeed each other with extraordinary rapidity and are causally related。 In this series the following elements may principally be distinguished。
（1） The physico…chemical process。
（2） The physiologico…sensory。
（3） The psychological。
（4） The physiologico…motor。
（5） The process of perception。''
It is not our task to examine the first four elements。 In order
clearly to understand the variety of perception； we have to deal with the last only。 I once tried to explain this by means of the phenomenon of instantaneous photographs （cinematographs）。 If we examine one such representing an instant in some quick movement； we will assert that we never could have perceived it in the movement itself。 This indicates that our vision is slower than that of the photographic apparatus； and hence； that we do not apprehend the smallest particular conditions； but that we each time unconsciously compound a group of the smallest conditions and construct in that way the so…called instantaneous impressions。 If we are to compound a great series of instantaneous impressions in one galloping step； we must have condensed and compounded a number of them in order to get the image that we see with our eyes as instantaneous。 We may therefore say that the least instantaneous image we ever see with our eyes contains many parts which only the photographic apparatus can grasp。 Suppose we call these particular instances a； b； c； d； e； f； g； h； i； j； k； l； m； it is self…evident that the manner of their composition must vary with each individual。 One man may compound his elements in groups of three： a； b； c；d； e； f；g； h； i； etc。； another may proceed in dyads： a； b；c； d；e； f；g； h；etc。； a third may have seen an unobservable instant later； but constructs his image like the first man： b； c； d；l； m； n； etc。； a fourth works slowly and rather inaccurately； getting： a； c； d；f； h； i；etc。 Such variations multiply； and when various observers of the same event describe it they do it according to their different characteristics。 And the differences may be tremendous。 Substitute numerals for letters and the thing becomes clear。 The relative slowness of our apprehension of visual elements has the other consequence that we interpolate objects in the lacun of vision _*according to our expectations_。 The best example of this sort of thing would be the perception of assault and battery。 When ten people in an inn see how A raises a beer glass against B's head； five expect： ‘‘Now he'll pound him；'' and five others： ‘‘Now he'll throw it。'' If the glass has reached B's head none of the ten observers have seen how it reached there； but the first five take their oath that A pounded B with the glass； and the other five that he threw it at B's head。 And all ten have really seen it； so firmly are they convinced of the correctness of their swift judgment of expectation。 Now； before we treat the witness to some reproach like untruth； inattention； silliness； or something equally nice； _*we_ had better consider whether his story is not true； and whether the difficulty might not really lie in the imperfection of our own sensory processes。 This involves partly what Liebmann has called ‘‘anthropocentric vision；'' i。 e。； seeing with man as the center of things。 Liebmann further asserts； ‘‘that we see things only in perspective sizes； i。 e。； only from an angle of vision varying with their approach； withdrawal and change of position； but in no sense as definite cubical； linear； or surface sizes。 The apparent size of an object we call an angle of vision at a certain distance。 But； what indeed is the different； true size？ We know only relations of magnitude。'' This description is important when we are dealing with testimony concerning size。 It seems obvious that each witness who speaks of size is to be asked whence he had observed it； but at the same time a great many unexpected errors occur； especially when what is involved is the determination of the size of an object in the same plane。 One need only to recall the meeting of railway tracks； streets； alleys； etc。； and to remember how different in size； according to the point of view of the witness； various objects in such places must appear。 Everybody knows that distant things seem smaller than near ones； but almost nobody knows what the difference amounts to。 For examples see Lotze； ‘‘Medical Psychology；'' Leipzig； 1852。
In addition we often think that the clearness of an object represents its distance and suppose that the first alone determines the latter。 But the distinctness of objects； i。 e。； the perceptibility of a light…impression； depends also upon the absolute brightness and the differences in brightness。 The latter is more important than is supposed。 Try to determine how far away you can see a key…hole when the wall containing the door is in the shadow； and when there is a window opposite the key…hole。 A dark object of the size of a key…hole will not be visib