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.On no other condition is its visibility possible.The conclusion is that things can properly be seenonly in what Lipps calls a surface, and that our knowledge of the third dimension must needs beconceptual, not sensational or visually intuitive.But no arguments in the world can prove a feeling which actually exists to be impossible.Thefeeling of depth or distance, of farness or awayness, does actually exist as a fact of our visualsensibility.All that Professor Lipps's reasonings prove concerning it is that it is not linear in itscharacter, or in its immediacy fully homogeneous and consubstantial with the feeling of literaldistance between two seen termini; in short, that there are two sorts of optical sensation, eachinexplicably due to a peculiar neural process.The neural process is easily discovered, in the caseof lateral extension or spreadoutness, to be the number of retinal nerve-ends affected by the light;in the case of pretension or mere farness it is more complicated and, as we have concluded, isstill to seek.The two sensible qualities unite in the primitive visual bigness.The measurement oftheir various amounts against each other obeys the general laws of all such measurements.Wediscover their equivalencies by means of objects, apply the same units to both, and translate theminto each other so habitually that at first they get to seem to us even quite similar in kind.Thisfinal appearance of homogeneity may perhaps be facilitated by the fact that in binocular visiontwo points situated on the prolongation of the optical axis of one of the eyes, so that the near onehides the far one, are by the other eye seen laterally apart.Each eye has in fact a foreshortenedlateral view of the other's line of sight.In The London Times for Feb.8, 1884, is an interestingletter by J.D.Dougal, who tries to explain by this reason why two-eyed rifle-shooting has suchadvantages over shooting with one eye closed.Get any book for free on: www.Abika.com THE PRINCIPLES OF PSYCHOLOGY191[78] Just so, a pair of spectacles held an inch or so from the eyes seem like one large medianglass.The faculty of seeing stereoscopic slides single without an instrument is of the utmostutility to the student of physiological optics, and persons with strong eyes can easily acquire it.The only difficulty lies in dissociating the degree of accommodation from the degree ofconvergence which it usually accompanies.If the right picture is focussed by the right eye, theleft by the left eye, the optic axes must either be parallel or converge upon an imaginary pointsame distance behind the plane of the pictures, according to tile size and distance apart of thepictures.The accommodation, however.has to be made for the plane of the pictures itself, and anear accommodation with a far-off convergence is something which the ordinary use of our eyesnever teaches us to effect.[79] These two observations prove the law of identical direction only for objects which excite thefoveæ or lie in the line of direct looking.Observers skilled in indirect vision can, however, moreor less easily verify the law for outlying retinal points.[80] This essay, published in the Philosophical Transactions, contains the germ of almost all themethods applied since to the study of optical perception.It seems a pity that England, leading offso brilliantly the modern epoch of this study, should so quickly have dropped out of the held.almost all subsequent progress has been made in Germany, Holland, and, longo intervallo,America.[81] This is no place to report this controversy, but a few bibliographic references may not beinappropriate.Wheatstone's own experiment is in section 12 of his memoir.In favor of hisinterpretation see Helmholtz, Phys.Opt., pp.737-9 ; Wundt, Physiol.Psychol., 2te Anfl.p.144;Nagel, Sehen mit zwei Augen, pp.78-82.Against Wheatstone see Volkmann.arch.f.Ophth., v.2-74, and Untersuchungen, p.286; Hering, Beiträge zur Physiologie, 29-45, also in Hermann'sHdbch.d Physiol., Ed.iii.1 Th.p.435; Aubert, Physiologie d.Netzhsut, p.322 ; Schön.Archivf.Ophthal., xxiv.1.pp.56-65; and Donders, ibid.xiii 1.p.15 and note.[82] When we see the finger the whole time, we usually put it in the line joining object and lefteye if it be the left huger, joining object and right eye if it be the right finger.Microscopists,marksmen, or persons one of whose eyes is much better than the other, almost always referdirections to a single eye, as may be seen by the position of the shadow on their face when theypoint at a candle-flame.[83] Professor Joseph Le Conte, who believes strongly in the identity-theory, has embodied thelatter in R pair of laws of the relation between positions seen single and double, near or far, onthe one hand, and convergences and retinal impressions, on the other, which, thoughcomplicated, seems to me by far the best descriptive formulation yet made of the normal facts ofvision.His account is easily accessible to the reader in his volume 'Sight' in the InternationalScientific Series, bk.ii c.3, so I say no more about it now, except that it does not solve any ofthe difficulties we are noting in the identity-theory, nor account for the other fluctuatingperceptions of which we go on to treat.[84] Naturally it takes a small object at a less distance to cover by its image a constant amount ofretinal surface.Get any book for free on: www.Abika.com THE PRINCIPLES OF PSYCHOLOGY192[85] Archiv f.Ophthal., Bd.xvii.Abth.2, pp.44-8 (1871).[86] A.W.Volkmann, Untersuchungen, p.253.[87] Philosophical Transactions, 1859, g.4.[88] Physiol.Optik, 649-664.Later this author is led to value convergence more highly.Arch.f.(Anat.u.) Physiol.(1878), p.322.[89] Anomalies of Accommodation and Refraction (New Sydenham Soc.Transl., London,1864), p.155.[90] These strange contradictions have been called by Aubert 'secondary' deceptions ofjudgment.See Grundzüge d.Physiologischen Optik (Leipzig, 1876), pp.601, 615, 627.One ofthe best examples of them is the small size of the moon as first seen through a telescope.It islarger and brighter, so we see its details more distinctly and judge it nearer.But because wejudge it so much nearer we think it must have grown smaller.Cf.Charpentier in Jahresbericht, x.430.[91] Revue Philosophique iii.9, p.220.[92] See Chapter XXIV.[93] The only exception seems to be when we expressly wish to abstract from particulars, and tojudge of the general 'effect
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