1 84 



NA TURE 



[June 20, 1S95 



§ 12. Each experiment on the slate and granite columns lasted 

 about two hours from the first application of heat and cold ; and 

 we generally found that after the first hour we could keep the 

 temperatures of the three junctions ver)- nearly constant. 

 Choosing a time of best constancy in our experiments on each 

 of the two substances, slate and granite, we found the following 

 results : — 



Slate : flux lines parallel to cleavage. 



f(T) = so°-2 C. 

 70l)= i23°-3- 



f(B) = 202°-3. 



The distances between the junctions were BM = 2'S7 cm. and 

 MT = 2'6 cm. Hence by the formula of § 2, 



>i<M. B) _ 7 31-^26 _ 281 _ 



Kl, M) 

 Aberdeen granite : 



79-0^257 307 



i'<T) = 8l°l. 

 f(B) =2i4°-6. 



0-91. 



The distances between the junctions were BM = I "9 cm. and 

 MT = 2'0 cm. 



^(MB) _ 645-=-20 _ 32^2 ^ .„„ 

 -t(TMJ 69-0-=- 1-9 363 



§ 13. Thus we see, that for slate, with lines of flux parallel to 

 cleavage planes, the mean conducti\-ily in the range from 123° C. 

 to 202" C. is 91 per cent, of the mean conductivity in the range 

 from 50' C. to 123° C, and for granite, the mean conducti\nty 

 in the range from 145° C. to 214" C. is SS per cent, of the mean 

 conductivity in the range from 81° C. to 145° C. The general 

 plan of apparatus, described above, which we have used only for 

 comparing the conductivities at different temperatures, will, we 

 l)clieve, be found readily applicable to the determination of 

 conductinties in absolute measure. 



THE RELATIOX liRTWEEN THE MOVE- 

 MENTS OF THE EYES AND THE MOVE- 

 MENTS OF THE HEAD} 



AVE all know that it was a long time before mankind found 

 out that the earth moves. For ages the apparent motion 

 of the heavenly Ixxlies was supposed to be their real motion, 

 the earth being fixed. We, who know something of the truth 

 in this matter, do not, however, any more than our ancestors 

 did, see or feel the earth move. We believe that it does so 

 either Ixsrause we have Ijecn told by some one who, we think, 

 knows about such things, or liecausc we have reasoned the 

 matter out from data observe<l by ourselves or reported by 

 credible olwcrvers. But in habitual thought and sjxjech we go 

 liack to the old assumption which, for our practical, terrestrial 

 purposes, answers well enough, and is perfectly in accordance 

 with our sensations. 



When we turn from the great Cosmos to the microcosm ; 

 when we compare the motion of our own body among the 

 v.irious fixed (terrestrially fixed) and moving bodies around us, 

 with the motion of the earth among the stars, we find quite a 

 different slate of matters. It never occurs to us that our own 

 l»ody is at rest, and that the trees, houses, \c., move. When 

 we really move we not only know, but feel and see that we are 

 moving, and every one learned or ignorant, old or young — if 

 only he is sober — feels and sees that the solid earth is fixed, 

 except on the rare occa.sion of an carthrjuake, and in the case 

 of .some illusions which we shall have to consider. I wish to 

 discuvs the cause of this sensation of the fixedness of the earth, 

 and also inciflenlally of the exception implied in the words I 

 have juM used, " if only he is sober." 



If we keep our head fixed and look at any really fixed scene 

 —say. a room in whi'-h there is nothing moving — or a landsca|x:, 

 ■' ■' railway trains, ships, moving Iwasts, 



'" '»' our eyes to run over it in as uniform 



"' •'• — >. • •' "•'• •'- >»<-• please, and sec that Ihc scene remains 



fixed. We might have supposed that, as we move our eyes 



• livcrcti Jjcfore the OxTortl 

 iiy Muwrum, Oxford, on 



I 



.M.. 



NO. 1338, VOL. 52] 



from right to left the whole scene, like a moving (xinorama^ 

 would seem to move from left to right, but it does not do so. 

 It remains visibly at rest, and we know, without any reasoning 

 about it, that the changes of view were produced by the motion 

 of our eyes. 



We fancy that we can move our eyes uniformly, that by a 

 continuous motion like that of a telescope we can move our 

 eyes along the sky-line in the landscape or the cornice of the 

 room, but we are wrong in this. However determinedly we try 

 to do so, what actually hapjiens is, that our eyes move like the 

 seconds hand of a w. itch, a jerk and a little pause, another jerk and 

 so on ; only our eyes are not so regular, the jerks are sometimes of 

 greater, sometimes of less, angular amount, and the pauses varj- in 

 duration, although, unless wc make an effort, they arc always short. 

 During the jerks we practically <lo not see at all. so thai we have 

 before us not a moving jxinorama, but a series of fixed pictures 

 of the same fixed things, which succeed one another rapidly. It 

 is not difficult to understand how this gives rise to a sensation of 

 the fixedness of the external scene. If, in the otherwise fixed 

 scene, there is a really moving object, we see it move, because 

 during the pauses, short .as they are, the moving object has 

 visibly changed its place, and in each of our fixed pictures the 

 moving object is seen to move. If it moves too slowly for this, 

 then we do not sec it move, but only infer its motion from com- 

 parison of its position at tlifferent times. If we keep our eyes 

 fixed on the moving object, and this is possible if it does not 

 move too fast or too irregularly, then we see it fixed and the 

 really fixed things moving, an illusion we have all observed 

 when the pier seems to move and the steamer remain at rest. 



That the eyes jerk in the way now stated can be made plain 

 by means of a simple ex|ieriment. If we have In the field of 

 view a bright object, such as an incandescent electric lamp, and 

 after running our eyes over the scene before us, shut our eyes, 

 we see secondar)- images of the bright object.' Now if the 

 eyes move continuously from one position to another, we should 

 see lx;tween the two secondary nnages of the bright object 

 corresponding to these two positions, a bright band compo.scd of 

 an infinite number of im.Tgcs each infinitely near its two neigh- 

 bours. But we see no such l»nd, but a finite numl)er of sharp 

 individual images, each of which corresponds to the position of 

 the eyes during a pause between jerks ; unless the bright object 

 is ver)' bright, there is nothing in the secondary image to repre- 

 sent the iX)sitions of the eyes during the jerk. If for a bright 

 object we take the sun, then we do see bands joining the sharp 

 seconilar)' images. These Ixinds are fainter than the sharp 

 images, and die away soimer. They are the impressions made 

 on the retina by the image of the sun jiassing rapidly across it 

 during the jerk. But, if with the fixed bright object in the 

 field we follow with our eyes a really moving thing, then on 

 shutting the eyes we see a band of light, because the image of 

 the bright object passed not very rapidly across the retina. 



This habit of jerking the eyes from one position of vision to 

 another, as fast as the light, wcll-jioised globes can 1k" .swung 

 round by the quick working, straight-filircd muscles which 

 move them, may be an innate habit, or it may have l>ecn ac- 

 quired by our looking at things and turning quickly from one 

 object of interest to another ; at all events, it is now the way in 

 which alone we can move them, unless we fix them on a moving 

 object. 



So far I have supposed the head fixed and the eyes alone 

 moving. Let us now attend to what happens when we move 

 our head.' 



The movement of the head, unless il is very rapid, makes no 

 difference at all in the phenomena just described. 



If we call the line along which we look during the jxiuse 

 between the two jerks a glance-line, we may describe the whole 

 phenomenon by saying thai the glance-lines are fixed relatively 

 to fixed external olijecis, whether the head is rotated ur not. 

 This, of course, means that, during a jiause, the eyes are rotated 

 relatively to the head :il>oul the axis about which the head is 

 really rotated, in the opjiosile sense and through the same .angle 

 OS the head. 



It might, for all that has lx:cn yet said, be supposed thai this 

 fixedness of the glance-lines, when the head is rotated, dei^nds 

 on the li.abit of looking at things ; but that this is mil ihe cause, 

 or, at all events, not the only cause, is plain from the fact that 

 the same relative movements of ihe eyes take place when wc 



_' The Mcondarj* im.lKes are iKIIcr seen if wc look at a white surface and 

 wink rapidly. 



'■i Bv " moving ihc head," I mean moving the head either alone or along 

 with ine body or anv p.iri .if it. 



