A new theory of vision : and other writings / [George] Berkeley ; introduction by A. D. Linsay.

  • Berkeley, George, 1685-1753.
Date:
[1910]
    h placitis consequi videtur, ipsum ad partes anticas apparens, ab intervallo longissime dissito, (quod et maximum sensibile quodvis intervallum quodammodo exsuperet) apparere. Ciim enim quo radiis minus divergentibus attingitur objectum, eo (seclusis utique praenotionibus et praejudiciis) longius abesse sentiatur, et quod parallelos ad oculum radios projicit, remotissime positam sestimetur: exigere ratio videtur, ut quod con- vergentibus radiis apprehenditur, adhuc magis, si fieri posset, quoad apparentiam elongetur. Quin et circa casum hunc generatim inquiri possit, quidnam omnino sit, quod apparentem puncti A locum deter- minet, faciatque quod constanti ratione nunc propius, nunc remotius appareat? Cui itidem dubio nihil quicquam ex hactenus dictorum analogid responderi posse videtur, nisi debere punctum A perpetub longissim^ semotum videri. Verum experientia secus attestatur, illud pro diversa oculi inter puncta B, Z, positione varie distans, nunquam fere (si unquam) longinquius ipso A libere spectato, subinde verb multb propinquius apparere ; quinimo, qub vergunt, eb speciem objecti propius accedere. Nempe, si puncto B admoveatur oculus, suo (ad lentem) fere nativo in loco conspicitur punctum A (vel seque distans, ad speculum); ad O reductus oculus ejusce speciem appropinquantem cernit; ad P adhuc vicinius ipsum existimat; ac ita sensim, donee alicubi tandem, velut ad Q, constitute oculo objectum summe propinquum apparens, in meram confusionem incipiat evanescere. Quae sane cuncta rationibus atque decretis nostris repugnare videntur, aut cum iis saltern parum amice conspirant. Neque nostram tantiim sententiam pulsat hoc experimentum, at ex aequo B 1-0 P
    caeteras quas norim omnes: veterem imprimis ac vul- gatam, nostrae prse reliquis affinem, ita convellere videtur, ut ejus vi coactus doctissimus A. Tacquetus isti prin- cipio (cui pen^ soli totam inaedificaverat Captoptricam suam) ceu infido ac inconstanti renunciarit, adeoque suam ipse doctrinam labefactarit; id tamen, opinor, minimS facturus, si rem totam inspexisset penitius, atque difficultatis fundum attigisset. Apud me verb non ita pellet hsec, nec eousque praepollebit ulla difficultas, ut ab iis quae manifest^ rationi consentanea video, discedam; praesertim quum, ut hie accidit, ejusmodi difficultas in singularis cujuspiam casus disparitate fundetur. Nimirum in praesente casu peculiare quiddam, naturae subtilitati involutum, delitescit, aegre fortassis, nisi perfectius ex- plorato, videndi modo, detegendum. Circa quod nil, fateor, hactenus excogitare potui quod adblandiretur animo meo, nedum plane satisfaceret. Vobis itaque nodum hunc, utinam feliciore conatu, resolvendum committo." IN ENGLISH AS FOLLOWS I '* I have here delivered what my thoughts have sug- gested to me, concerning that part of optics which is more properly mathematical. As for the other parts of that science (which being rather physical, do conse- quently abound with plausible conjectures, instead of certain principles) there has in them scarce anything occurred to my observation, different from what has been already said by Kepler, Scheinerus, Descartes, &c. And, methinks, I had better say nothing at all, than repeat that which has been so often said by others; I think it therefore high time to take my leave of this subject. But before I quit it for good and all, the fair and ingenuous dealing that I owe both to you and to truth, obliges me to acquaint you with a certain untoward difficulty, which seems directly opposite to the doctrine I have been hitherto inculcating, at least, admits of no solution from it. In short it is this. Before the double convex glass or concave speculum E B F, let the point A be placed, at such a distance that the rays proceeding
    from A, after refraction or reflection, be brought to unite somewhere in the ax A B. And suppose the point of union {i.e. the image of the point A, as hath been already set forth) to be Z; between which and B, the vertex of the glass or speculum, conceive the eye to be any where placed. The question now is, where the point A ought to appear. Experience shows, that it doth not appear behind at the point Z, and it were contrary to nature that it should; since all the impression which affects the sense comes from towards A. But from our tenets it should seem to follow, that it would appear before the eye at a vast dis- tance off, so great as should in some sort surpass all sensible dis- tance. For since, if we exclude all anticipations and prejudices, every object appears by so much the further off, by how much the rays it sends to the eye are less diverging ; and that object is thought to be most remote, from which parallel rays proceed unto the eye; reason would make one think, that object should appear at yet a greater distance, Moreover it may in general be asked concerning this case, what it is that determines the apparent place of the point A, and maketh it to appear after a constant manner, sometimes nearer, at other times further off? To which doubt I see nothing that can be answered agreeable to the principles we have laid down, except only that the point A ought alw^ays to appear extremely remote. But on the con- trary, we are assured by experience, that the point A appears variously distant, according to the different situations of the eye between the points B and Z. And that it doth almost never (if at all) seem further off, than it would if it were beheld by the naked eye; but on the contrary, it doth sometimes appear much nearer. B l-o P
    Nay, it is even certain, that by how much the rays falling on the eye do more converge, by so much the nearer does the object seem to approach. For the eye being placed close to the point B, the object A appears nearly in its own natural place, if the point B is taken in the glass, or at the same distance, if in the speculum. The eye being brought back to O, the object seems to draw near; and being come to P, it beholds it still nearer: and so on by little and little, till at length the eye being placed somewhere, suppose at Q, the object appearing extremely near, begins to vanish into mere confusion. All which doth seem repugnant to our principles ; at least, not rightly to agree with them. Nor is our tenet alone struck at by this experiment, but likewise all others that ever came to my knowledge are every whit as much endangered by it. The ancient one especially (which is most commonly received, and comes nearest to mine) seems to be so effectually overthrown thereby, that the most learned Tacquet has been forced to reject that principle, as false and uncertain, on which alone he had built almost his whole Catoptrics, and consequently by taking away the foundation, hath him- self pulled down the superstructure he had raised on it. Which nevertheless I do not believe he would have done, had he but considered the whole matter more thoroughly, and examined the difficulty to the bottom. But as for me, neither this, nor any other difficulty shall have so great an influence on me, as to make me re- nounce that which I know to be manifestly agreeable to reason. Especially when, as it here falls out, the difficulty is founded in the peculiar nature of a certain odd and particular case. For in the present case some- thing peculiar hes hid, which being involved in the subtilty of nature, will perhaps hardly be discovered till such time as the manner of vision is more perfectly made known. Concerning which, I must own, I have hitherto been able to find out nothing, that has the least show of probability^ not to mention certainty. I shall therefore leave this knot to be untied by you, wish- ing you may have better success in it than I have had." XXX. The ancient and received principle, which
    Dr. Barrow here mentions as the main foundation of Tacquet's Catoptrics, is, that 'every visible point seen by reflection from a speculum, shall appear placed at the intersection of the reflected ray and the perpendicular of incidence:' which intersection in the present case happening to be behind the eye, it greatly shakes the authority of that principle, whereon the aforementioned author proceeds throughout his whole catoptrics, in determining the apparent place of objects seen by reflec- tion from any kind of speculum. XXXI. Let us now see how this phenomenon agrees with our tenets. The eye the nearer it is placed to the point B in the above figures, the more distinct is the appearance of the object: but as it recedes to O, the appearance grows more confused; and at P it sees the object yet more confused; and so on, till the eye being brought back to Z, sees the object in the greatest confusion of all. Wherefore by Sect. xxi. the object should seem to approach the eye gradually, as it recedes from the point B, viz. at O it should (in consequence of the principle I have laid down in the aforesaid section) seem nearer than it did at B, and at P nearer than O, and at Q nearer than at P; and so on, till it quite vanishes at Z. Which is the very matter of fact, as any one that pleases may easily satisfy himself by experiment. XXXII. This case is much the same, as if we should suppose an Englishman to meet a foreigner, who used the same words with the English, but in a direct con- trary signification. The Englishman would not fail to make a wrong judgment of the ideas annexed to those sounds, in the mind of him that used them. Just so in the present case, the object speaks (if I may so say) with words that the eye is well acquainted with, viz. confusions of appearance; but whereas heretofore the greatest confusions were always wont to signify nearer distances, they have in this case a direct contrary signi- fication, being connected with the greater distances. Whence it follows, that the eye must unavoidably be mistaken, since it will take the confusions in the sense it has been used to, which is directly opposed to the true.