PERCEPTION. 



PERCUSSION. 



Ml 



they are totally destitute of foundation. A tound theory mint assign 

 real and not imaginary causes for the phenomena which it professes to 

 explain; and such causes mint have a manifest competency to the 

 ajieto ascribed to them. But the hypotheeei in question entirely 

 want both of the** tmentiils Who can prove the ezutnce of the 

 animal spirit* of Dec Cartes, or the vibration* of Hartley ; or, granting 

 their eiistmc*. who can ahow any correspondence between them and 

 the formation of image* in the brain f All we can affirm with certainty 

 T""**"*; the mean* of perception in, that, under certain circuin- 

 tanoM, that is, when an impreaaion U made on the bodily organs and 

 communicated by the nerre* to the brain, perception takes place. The 

 impressions *o communicated are the occasion* of the raind perceiving, 

 but we can assign no reason why it should do so under them circum- 

 stance* invariably, and not under any other, further than that such is 

 the constitution of our nature. 



If the act of perception be examined, it will be found that we obtain 

 by it a certain amount of information respecting the object perceived. 

 We discover that it ha* particular qualities, a* for example, that it U 

 extended, that it ha* figure, that it i* hard or soft, rough or umooth, 

 *c. The notion thus formed may vary in respect of distinctness in all 

 possible degrees. In the light of twilight a body is discerned more 

 obscurely than in the full light of noon day ; and more obscurely still 

 in proportion a* the darkness deepens. The notion we get of an object 

 by perception is accompanied by an irresistible and immediate con- 

 viction of its real existence. An object may indeed be perceived so 

 indistinctly a* to leave us in doubt whether it be real or not. If it be 

 very distant, or involved in darkness, this may happen. But when it 

 is plainly perceived, there is, along with the perception, a perfect 

 conviction of it* reality. We can no more doubt of it* existence than 

 we can of our own. And this conviction is immediate. It is not the 

 result of a process of reasoning founded on our perception*, but inse- 

 parably connected with them, and a* instantaneous as the assent 

 rendered to axiomatic truth*. It may be also remarked that the belief 

 in the existence of the object* of perception is not more immediate 

 and deeply rooted than is the belief that they exist externally to ua. 

 They do not seem to have their place in the mind itself, but to exist 

 independently of it altogether. These statements accord with the 

 universal experience of mankind, and may be verified by all who 

 to bestow the slightest attention on the intimations of con- 



It would be a tedious as well as a useless task to dwell minutely on 

 the numerous theories that have been framed of perception. In certain 

 important particulars nearly all of them coincide; while in others, 

 equally if not more important, they are for the most part at variance. 



Democritus taught that perception was the result of the impressions 

 made on the organ* of sense by images (f}Aa), which constantly 

 emanated from bodies, and varied according to the conformation of 

 their original*. (Plut. ' Plac. Phil./ 1. iv., ch. 8, Ac.) 



Plato, in the seventh book of hi* ' Republic ' (ad init.) illustrate* the 

 manner in which we perceive objects, by the figure of a cave, in which 

 men lie bound, so that they can turn their eyes only to one part of it, 

 where rays from a distant light stream in, and shadows of bodies, 

 supposed to pass between them and the light, are beheld, the bodies 

 themselves being invisible. He thus conceived that we perceive only 

 the shadow* of things, and not things themselves. 



This opinion of Plato was substantially the same with that of his 

 scholar Aristotle, and of the Peripatetics generally. Aristotle ('De 

 An..' L iii., c. S, 3) taught that a* the senses :annot receive material 

 objects themselves, they receive their images. These images are the 

 only objects of perception to the mind. As impressed upon the senses, 

 they are termed sensible species ; more spiritualised, they become 

 object* of memory and imagination, and are termed phantasm* ; still 

 further refined, so a* to be objects of science, they are named intel- 

 ligible specie*. 



The theory of Epicurus was little other than a modification of that 

 of Aristotle. He supposed that bodies are continually sending off 

 from their surfaces slender films or spectres of such subtlety that they 

 easily penetrate by the seme* to the brain. (Lucret., L iv., v. 34, 

 ,*c.) 



Locke employs an ilUutration of the manner of perception that 

 appears to have been borrowed from that of Plato : " Methink*," be 

 says, " the understanding is not much unlike a closet wholly shut from 

 light, with only some little light opening left to let in external visible 

 resemblances, or ideas of things without Would the pictures coming 

 into a dark room but *Uy there, and be so orderly si to be found 

 upon occasion, it would very much resemble the understanding 

 of a man in reference to all objects of sight and the ideas of them." 



The similitude of Locke, or rather of Plato, may be applied to all 

 the systems of perception that have ever been formed, by merely 

 substituting ideas, and, in the case of Hume, impression*, for what 

 were anciently denominated species snd pbantanms. All these theories 

 agree in maintaining that images are the only immediate objects of 

 perception to the mind. Hume, Berkeley, and others indeed hold that 

 these are the exclusive object* ; but the common hypothesis admit* 

 the existence of things of which these are but the representatives, and 

 which we mediately discern. It may be sufficient to remark con- 

 cerning these opinions, that they are diametrically opposed to the 

 testimony of our own consciousness. Instead of Informing us that 



image* alone are the direct object* of our perception, consciousness 

 intimate* nothing respecting images at all. Unless its representations 

 are altogether deceptive, it i* not things within the mind, but things 

 external to it, that we perceive ; not image* of object*, but the very 

 object* themselves. This is testimony to which we yield instinctive 

 credence. It is too cogent and unquestionable to be set aside by 

 reasoning of any kind, far lea* by reasoning based upon certain 

 imagined relations subsitting between matter and spirit which we are 

 incapable of apprehending, and the application to mind of law* which 

 apply solely to the object* of physical investigation. 



One observation, intentionally deferred, remains still to be made 

 respecting perception, namely, that it i* greatly modified by habit and 

 by the cultivation and development of the other powers. Thus the 

 perceptions of a man and those of a child, both contemplating a piece 

 of complex machinery, the one being aware of its principles and 

 arraagements, the other completely ignorant of them, must in some 

 respects considerably differ. In like manner the perception* of a blind 

 man, by means of those organs of sense which are unimpaired, are 

 distinguished in many particulars from those of the individual who has 

 never been without the faculty of vUion. Numerous instance* of a 

 similar kind might easily be specified. A full account of acquired per- 

 rr|,tion, such as those alluded to, is still a desideratum in this depart- 

 ment of philosophical inquiry. 



PERCH or POLE. This measure, though now mostly used a* a 

 square measure (a perch usually meaning a square perch, or a square 

 of a perch in length and breadth), was originally a measure of length, 

 arising out of the custom of measuring small portions of land by a stalV 

 or pole. The word prrtica, from which it comes, means a wooden staff. 

 But the pole with which land was measured not only differed very 

 much in different countries, but in different parts of the same country. 

 Ducange and hi* editors find records of the use, in different parts of 

 England and France, of perches of 7}, 10 (the oldest English), 12, 15, 

 16, 18, 20, 21, 22, 25, and 27 feet. Here i* but another instance (a* 

 in MILE and LEAGCE) of the tendency of measures to lengthen. Forty 

 perches (quarantena), under the name of forty-long or furlong, became 

 a common measure of length ; and a piece of land forty perches long 

 and one deep, was called perticata. (Spelman, cited by Ducange.) 

 Thus the perch and furlong, though afterwards received into the sys- 

 tem of measures which ends with the mile, originally formed no part 

 of it whatever. [Miu.j 



By an early statute, entitled ' Compositio Ulnarum et Perticarum,' 

 the perch was fixed at 164 feet, or five yards and a half; and four 

 perticata) (though that word is not used) were defined as an acre. 



A perch is the quarter of a chain, and a square perch contains 3(1$ 

 square yards. 



PERCHLORIC ACID. [CHLORINE.] 



PERCHLORINATED QUINONE. [QpixoNE.] 



PERCHLOROXYNAPHTHALIC ACID (C^HCIA). Chloroxy- 

 naphthalene acid. When perchloroxynaphthyl is placed in contact 

 with potash it is instantly transformed into a magnificent carmine- 

 coloured body, which is the potash Bait of this acid. The acid itself is 

 yellow, insoluble in water, but soluble in boiling alcohol. The ammonia 

 salt, which is of a bright red, is, like the potash salt, quite insoluble 

 in water. This acid may be regarded as pentachlorinated alizarine. 



PKRCHLORRUBIAN. [MADDER, COLOURING MATTERS OP.] 



PERCUSSION, CENTRE OF, the point of a system which moves 

 about a fixed axis, at which a force may act in such a manner as to 

 produce no pressure on the axis. Its distance from the axis is the 

 same as that of the centre of oscillation. [OSCILLATION,] 



PERCUSSION, in medicine, is the method of eliciting sounds by 

 Btriking the surface of the body, for the purpose of determining the 

 condition of the organs subjacent to the parts struck. 



This means of diagnosis was first employed by Avenbrugger in the 

 middle of the last century ; it was afterwards extensively adopted by 

 Corvisart in investigating the diseases of the heart, but its value, like 

 that of all the other branches of auscultation, was not fully appreciated 

 till Laennec made them the subject of his peculiar study. Since 

 his time, it* value ha* been considerably enhanced by the labours of M. 

 Piorry. 



Everybody knows that when a hollow body is struck, there i* a 

 vibration produced in the air within it, which being communicated 

 through the walls to the external air, produces a ringing Round, whose 

 tone varies with the size of the hollow body, the material of which it 

 is composed, and many other circumstances ; but that if the same 

 body be filled with a fluid or a nearly solid substance, no other sound 

 is produced than that which result* from the striking together of two 

 x.'liil bodies of the same materials as the wall of the cavity and the 

 substance with which it is struck. On these circumstances the practice 

 of percussion is based. If any part of the body beneath which there 

 is a hollow organ, or one containing air in tubes, be struck, a 

 resonance is produced ; if any part which lies over a solid or a fluid be 

 similarly struck, the sound emitted is merely the dull noise of two 

 solid and rather soft bodies. 



The modes of employing percussion are various. The simplest and 

 most convenient is to place one finger of the left hand flat upon the 

 part to be examined, and to strike it lightly, but rather sharply, with 

 the ends of the three first finger* of the right hand set close together 

 on the same level. Instrument* called plexiuieters have been invented 



