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But here again the limit of the application of the term ‘‘illusion'' is difficult to indicate。 That distant things seem to be smaller than they are； that railway tracks and two sides of a street seem to run together are intrinsically real illusions of sense； but they are not so calledthey are called the laws of perspective； so that it would seem that we must add to the notion of sense… perception that of rarity； or extraordinary appearance。
I have found still another distinction which I consider important。 It consists in the difference between real illusions and those false conceptions in which the mistake originates as false inference。 In the former the sense organ has been really registering wrongly； as when； for example； the pupil of the eye is pressed laterally and everything is seen double。 But when I see a landscape through a piece of red glass； and believe the landscape to be really red； the mistake is one of inference only； since I have not included the effect of the glass in my concluding conception。 So again； when in a rain I believe mountains to be nearer than they really are； or when I believe the stick in the water to be really bent； my sensations are perfectly correct； but my inferences are wrong。 In the last instance； even a photograph will show the stick in water as bent。
This difference in the nature of illusion is particularly evident in those phenomena of expectation that people tend to miscall ‘‘illusions of sense。'' If； in church； anybody hears a dull； weak tone； he will believe that the organ is beginning to sound； because it is appropriate to assume that。 In the presence of a train of steam cars which shows every sign of being ready to start you may easily get the illusion that it is already going。 Now； how is the sense to have been mistaken in such cases？ The ear has really heard a noise； the eye has really seen a train； and both have registered correctly； but it is not their function to qualify the impression they register； and if the imagination then effects a false inference； that can not be called an illusion of sensation。
The incorrectness of such classification becomes still more obvious when some numerical； arithmetical demonstration can be given of the presence of faulty inference。 For example； if I see through the window a man very far away clearing a lot with an ax； I naturally see the ax fall before I hear the noise of the blow。 Now； it may happen that the distance may be just great enough to make me hear the sound of the second blow at the moment in which I see the delivery of the third blow。 Thus I perceive at the same moment； in spite of the great distance； both the phenomena of light and of sound； just as if I were directly on the spot。 Perhaps I will wonder at first about these physical anomalies； and then； if I have made my simple mistake in inference； I shall tell somebody about the remarkable ‘‘sensory illusion'' I had today； although no one had ever supposed me capable of being deceived in this way。 Schopenhauer calls attention to the familiar fact that on waking after a short nap all localizations are apparently perverted； and the mind does not know what is in front； what behind； what to the right； and what to the left。 To call also this sensory illusion； would again be wrong； since the mind is not fully awake； and sufficiently orientated to know clearly its condition。 The matter is different when we do not properly estimate an uncustomary sense…impression。 A light touch in an unaccustomed part of the body is felt as a heavy weight。 After the loss of a tooth we feel an enormous cave in the mouth； and what a nonsensical idea we have of what is happening when the dentist is drilling a hole in a tooth！ In all these cases the senses have received a new impression which they have not yet succeeded in judging properly； and hence； make a false announcement of the object。 It is to this fact that all fundamentally incorrect judgments of new impressions must be attributed；for example； when we pass from darkness into bright light and find it very sharp； when we find a cellar warm in winter that we believe to be ice…cold in summer； when we suppose ourselves to be high up in the air the first time we are on horseback； etc。 Now； the actual presence of sensory illusions is especially important to us because we must make certain tests to determine whether testimony depends on them or not； and it is of great moment to know whether the illusions depend on the individual's mind or on his senses。 We may trust a man's intellect and not his senses； and conversely； from the very beginning。
It would be superfluous to talk of the importance of sensory illusion in the determination of a sentence。 The correctness of the judgment depends on the correctness of the transmitted observations； and to understand the nature of sense…illusion and its frequency is to know its significance for punishment。 There are many mistakes of judges based entirely on ignorance of this matter。 Once a man who claimed； in spite of absolute darkness； to have recognized an opponent who punched him in the eye； was altogether believed； simply because it was assumed that the punch was so vigorous that the wounded man saw sparks by the light of which he could recognize the other。 And yet already Aristotle knew that such sparks are only subjective。 But that such things were believed is a notable warning。＇1＇
＇1＇ For literature of Edmund Parish： ber Trugwahrnehmung。 Leipsig 1894。 A Cramer： Geriehtliche Psychiatrie。 Jena 1897。 Th。 Lipps： sthetische Eindrcke u。 optische。 Taschung。 J。 Sully： Illusions； London； 1888。
Section 99。 （2） _Optical Illusions_。
It will be best to begin the study of optical illusions with the consideration of those conditions which cause extraordinary； lunatic images。 They are important because the illusion is recognizable with respect to the possibility of varied interpretations by any observer； and because anybody may experiment for himself with a bit of paper on the nature of false optical apprehension。 If we should demonstrate no more than that the simplest conditions often involve coarse mistakes； much will have been accomplished for the law； since the ‘‘irrefutable evidence'' of our senses would then show itself to need corroboration。 Nothing is proved with ‘‘I have seen it myself；'' for a mistake in one point shows the equal possibility of mistakes in all other points。
Generally； it may be said that the position of lines is not without influence on the estimation of their size。＇2＇ Perpendicular dimensions are taken to be somewhat greater than they are。 Of two crossed lines； the vertical one seems longer； although it is really equal to the horizontal one。 An oblong； lying on its somewhat longer side； is taken to be a square； if we set it on the shorter side it seems to be still more oblong than it really is。 If we divide a square into equal angles we take the nearer horizontal ones to be larger； so that we often take an angle of thirty degrees to be forty…five。 Habit has much influence here。 It will hardly be believed； and certainly is not consciously known； that in the letter S the upper curve has a definitely smaller radius than the lower one； but the in