July 17, 1885.] 



KNOWLEDGE 



Ul^^^ AN ILLUSTRATED ^J^ 



II >^ MAGAZINE OF SqENCE^' 



PLAINCrWORDED-EXACTLYDESCRIBED 



LONDON FRIDAY, JULl 17, 1885 



Contents < 



nlnsioDB of the Senses. By H. A. 

 Life in "VeMh. By' Williim 



Soventeen-'Tear Cicadal'ViHus.') ' By 



C.V.Riley 42 



Mysteries and Moralities. II. By 



Edward Clodd 44 



Hnddy EcUpse of the Moon. By K. 



A. Proctor 45 



The Tonne Electrician, (ntut.) 



ByW.B&go 47 



Onr Household Insects. (lUui.) 



ILLUSIONS OF THE SENSES. 



By Rici 



) A. Proctoe. 



PROFESSOR LE CONTE of the California Univt 

 sity has recently pnblished in the North A', 

 Review an interestinj^ paper on the Evidence of the 

 Senses, in which he shows that on the one hand the 

 senses often afiord most incorrect information while on 

 the other the jiowers of .such scientific instruments as 

 give exact information would be utterly unsuitable sub- 

 stitutes for our less exact senses. Sight tells us that an 

 object is flat when it is round, touch that an object is 

 double when it is single, hearing that sounds come from 

 close by when they really reach us from a great distance ; 

 but on the other hnnd to liuvc eyes with telescopic power, 

 or fingers as Kuiisltivc as a chemist's balance, or ears with 

 the sound-gathering (luiilities of the microphone, would 

 unfit us for the kind of life we have to lead upon this 

 work-a-day world of ours. 



I propose now to discu.ss the question dealt with by 

 Lo Conte, with special reference to the liability of ouj 

 senses to various forms of error. Taste and smell need 

 not here occupy our attention. 

 the other senses in scientific 

 the purposes to which they 

 cerned they 



i used tha 

 research ; and so far as 

 e chiefly directed arc con- 

 : trustworthy. They may 



senting as plef 

 leterious, but once experience has determined the qualities 

 and effects of substances having such and such t:i«tc or 

 odour, we are not often deceived in identifying those 

 substances thereafter. 



The sense of touch is commonly understood as in- 

 cluding the sense of heat-effects. But here, as Reid long 

 since pointed out, our division of the senses is unsound. 

 Undoubtedly the sense of touch is entirely distinct from 

 the sense of heat, — though we may be s;iid to fed in both 

 cases. The error probably arose from the circumstance 

 that the same organs seem employed in noting the effects 

 of contact and the effects of boat. I touch a surface to 

 see if it is hard or soft, rough or smooth, just as I touch 

 a surfiico to see if it is hot or cold : moreover there is no 

 part of the body which is sensible to the effects of con- 



f „t «lii.li 1^ i.nt ^l^<. MiiMlil- t<i tin ( ff( cts of heat and 

 ,,,M I III \ I I _iii I irniilMl 1 iHm I nco between the 

 s, II ( I f t IK 1 1 1 1 II tin ii| . f ill ii iiL'uc IS employed 

 f, , tlK 1,1 i.Hut I i1m 1 I I I f tnu h, md the Fcns. of 

 t.s(, ,vtllH .hllMMHi iHtui.u tisti, and t.iiKh isnot 

 niou'iuiikdl til 111 till .lilliKntc between he it md 

 ton, h 



Till 11 fi 11 m dt ihng ^m<1i criors iffectmg the evidence 

 _i\.n 1\ ilu suise of toueh I cemsidtr only those really 

 1. litiii_ t till eliects nf cent i^t, deihng stpaiately with 

 tliciM ic htiiig t(i the effects of he xt and cold 



Aristotle long smce pointed out how the sense of touch 

 may be deceived when the organs of touch are employed 

 in some unaccustomed manner. It was he who first 

 mentioned, if he did not invent, the experiment of roll- 

 ing a pea between the tijis of the first and second fingers, 

 after the second finger has been crossed over the first. 

 This experiment is instructive as showing how much of 

 the significance of the teachings of our senses may be due 

 to the eiiect of long-continued training. Every time 

 we touch with the finger-tips an object of known shape, 

 we are in reality tenching our fingers that such and such 

 impressions have such and such a meaning. When two 

 fingers are crossed, the finger-tips receive different im- 

 pressions from those which they receive in their normal 

 position, and we naturally misinterpret the meaning 

 of the impressions so received. Thus if I touch 

 with my first and second fingers the sides of a 

 space shaped thus ^ , the outsides of the fingers 

 come in contact with the curved surface, whereas 

 the insides of the fingers feel such a surface as this, — 

 '^ : so soon as the fingers are crossed these effects are 

 reversed ; the outsides of the fingers are brought together 

 by the crossing and touch a surface shaped thus '^ , 

 telling us apparently that it is really a surface shaped 

 thus ^ that we are touching. To test this apply the 

 crossed fingers to a surface shaped C"^ < so that the 

 fingers touch the convex curves near their place of 

 meeting : now we fijid that we no longer seem to be 

 touching two curves, but one. It must be admitted, 

 however, that this experiment is less striking than the 

 other; the information conveyed by the finger-tips 

 instead of seeming definitely and decidedly incorrect, 

 appears but vaguely erroneous. 



Let us try a few other experiments with crossed 

 fingers. Take a penholder or pencil, and with first and 

 second fingers crossed slide the finger-tips along the 

 pencil or holder. If the eyes are closed the fingers 

 seem to tell us emphatically that we are feeling two 

 I.arallel rods. Yet if the eyes are directed to the finger- 

 tips the illusion disappears. This is not, however, 

 because tlii' eyes assure us that there is but one pen or 

 pencil ; it is iiecause the eyes show us that the fingers 



1 not 

 1 fingers the 



wiedg. 



I, feel 



save us fr^ni lli 



tip of the nose. AN'e know certainly t 



single nose-tip ; yet the absurd and illusory feeling that 



we have two noses is immediately produced. The 



illusion is strengthened if tlie crossed finger-tips are 



caused to slide \ilong the ridire of the nose. Very 



.■urii.us illusions are" iToilueeil if the cross.'d finger- 



Wl ' 



■\d,^r 



little liii-i r. O.olh being ImiH someuliat to«ards the 

 palm, so that tlie second or third fingers are behind 

 them) and then feeling with these crossed fingers the 



