Erect vision from an inverted image / by B.F. Joslin.

  • Joslin, Benjamin F. (Benjamin Franklin), 1796-1861.
Date:
[1845]
Catalogue details

Licence: Public Domain Mark

Credit: Erect vision from an inverted image / by B.F. Joslin. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    TO Urmation of this law, that the ocular spectrum seen on shutting the c after viewing a black paper figure of a man held up in the direclio; l; the sun, appeared erect or inverted according to the position of theobj n. He also adduces the fact that “ if we press the eyeball at any ] in where the retina is, we shall see the luminous impression which is ] duced, in a direction perpendicular to the point of pressure.” On th would remark, that when mechanical pressure on the eyeball produ k the perception of light, the direction of the imaginary external lumin iioi point from which the light appears to proceed, must appear to be loe same as that from which light must have actually proceeded, in orde tli fall on the part of the retina mechanically pressed. Whether it be an ac m luminous impression from without, or mechanical pressure generating i tei tation or light within, the law which I have stated applies ; viz., the ,il of connexion between apparent direction and the locality of impressi je The retina being so constituted as ordinarily to give no perception it that of light; then if a pin-head excites it through the other coats, o isi star through the humors, in either case there appears a luminous poi ar and this point, whether real or imaginary, must appear in the same dir ef tion, if the impressed point of the retina is in both cases the sa.me. isi The same law of locality of impression applies to another of b David Brewster’s supposed confirmations of this law of visible direct eoi by which he explains erect vision. The fact which he adduces is, t los pressure on the eyeball changes the apparent direction of objects. ♦] io explanation is that such pressure “alters the spherical form of the 5 e] face of the retina,” and consequently the direction of its perpendicul; ip which we regarded as the lines of visible direction ; and alters their pi; ii of meeting which is regarded as the centre of visible direction. di In order to test this theory, I resorted to a simple experiment, wh li any one can easily repeat, and by which the theory is refuted. I pli a candle at a short distance before me, and look at a more distant obj ri beyond it. Then, as the light falls on dissimilar parts of the two retii:,; I see two flames. If I then press the right eyeball with the finger; jc left flame, which is that seen by the right eye, changes its apparent })la se Now the problem is to determine whether this change of apparent ph ct is attributable to a change in the location of the image on the retina,■fti to a change in the form of the eyeball. In one respect, the problem i.|ii little complicated, inasmuch as a change in the form of the retina vvoi ol generally be attended by some change in the locality of the part i ii pressed, as well as by a change in the direction of its perpendicular ; jc that even were the axis of vision unchanged by the pressure. Sir Davi a[ explanation would not be confirmed by the eflect of pressure in changi re the apparent direction. The effect might still be—and so far as deper e on change of form undoubtedly is—owing to change of locality in i li image. But in order to remove all ambiguity arising from this soun ij and to render it evident that the change of direction in the object p p duced by pressure is not chiefly attributable to a change of formli‘ the retina, I modify the foregoing experiment in the following marine i) Instead of forcibly pressing on the eyeball, I rub the finger gently o^ rt it, so as to rotate the eyeball without sensibly changing its form. T ^ finger being gently pressed on either eyelid, is drawn over the eyeb i. parallel to its surface. The consequence is that the change in the ap} ?,3 rent position of the flame is much greater than that which results from t most violent pressure. Such a result is wholly inexplicable by Sir Da' 1.
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