The Philosophical magazine; a journal of theoretical, experimental and applied physics . true direction of points would have coincided, and objectswould have changed neither their form nor their position du-ring the motion of this hypothetical eyeball round the commoncentre of the two coats. But as such an eye could not haveafforded sufficiently distinct vision, ihe introduction of thecrystalline lens became necessary; and it is owing to the se-condaiy refractions at its surfaces and within its mass of vari-able density, that the parallax of visible direction is produced. The following experim
The Philosophical magazine; a journal of theoretical, experimental and applied physics . true direction of points would have coincided, and objectswould have changed neither their form nor their position du-ring the motion of this hypothetical eyeball round the commoncentre of the two coats. But as such an eye could not haveafforded sufficiently distinct vision, ihe introduction of thecrystalline lens became necessary; and it is owing to the se-condaiy refractions at its surfaces and within its mass of vari-able density, that the parallax of visible direction is produced. The following experiment will establish the existence, andexplain the nature of this parallax. Let M N, fig. 1, be theeyeball, C the centre of curvature of the retina, and also thecentre of motion of the eyeball. Having placed an opakescreen S several inches from the eye, till its inner edge just* Opuscules Mathanatiqucs, torn. i. mem. ix. p. 266. in Simle mid Binocular Vision. 359 eclipses a luminous object A, look away from the screen, andthe object A will appear. Keeping the head steady, place Fig. another screen S* so that, when viewed directl}, it does noteclipse another luminous object B, the line C S B just grazingthe outer edge of B. When the screens and luminous objects,therefore, are so arranged that A is invisible vvhen the axis ofthe eye is directed to S or to A, and B visible when the axisof the eye is directed to S or B,—then by turning the eyefrom A to B, A will appear^ and B will disappear, exhibitingthe curious effect of an invisible body appearing by lookingavoay from it, and of a visible body disappearing by lookingat it! Had the eyeball M N been our hypothetical one, theseeffects would not have been produced. All objects, near andremote, would have retained their relative positions and mag-nitudes during its rotation. Hence it follows, that we are not entitled to reject any lawof visible direction, because it gives a position to visible ob-jects different from their real p
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