NATURAL PHILOSOPHY. 151 



ments that the impression of itself is not essentially suggestive of the special 

 retinal surface on which it is received. 



Exp. 1. Let a short tube of black pasteboard, one-fifth of an inch in diam- 

 eter, be fixed in a hole in the centre of a large sheet of the same material. 

 Hold the sheet a few inches before the face of a second person, and between 

 him and a bright window, moving it to and fro until the bright circular aper- 

 ture of the tube is brought directly in front of one of the eyes, suppose the 

 left eye; and let him fix his attention upon the sky or cloud to which the tube 

 is directed. He will feel as if the impression or image of the hole belongs 

 equally to both eyes, and will be unable to determine w r hich of them really 

 receives the light. 



Exp. 2. Similar results may be obtained by rolling half a sheet of letter 

 paper into a tube, of about one inch in diameter, and holding it before and a 

 little in advance of one eye, while both are directed to a white wall some feet 

 distant. Keeping the view fixed upon the wall, there will be seen upon its 

 surface a circular image of the remote aperture of the tube. This, as we 

 look intently at it, will appear as if seen equally by both eyes, occupying a 

 midAvay position between them. If now the eyes be converged to some point 

 nearer than the end of the tube, the circular image will appear against the 

 side of the tube, giving the impression that it is seen by the eye which is 

 remote from the tube, and is, at the same time, directed towards the outside. 

 For the complete success of this experiment, the wall shoiild be only moder- 

 ately bright, and but little light should fall on the exterior of the tube next 

 the uncovered eye. 



Exp. 3. Let two tubes of stiff paper, each one inch in diameter and six 

 inches long, be held close to the two eyes in a converging direction so that 

 the outer ends may touch each other. Then directing the view through 

 them to a white wall, at a short distance, the observer will see the two tubes 

 as one, with a single circular opening clearly marked out on the wall. If 

 now a small object, as the end of the little finger, be brought near and in 

 front of one of the tubes, it will take its place within this circle, and will 

 seem to be equally an object of vision to both eyes, so that the observer will 

 be wholly unable to decide before which eye it is actually placed. 



Let the observer next direct his view to a very remote object, as the sky, 

 seen through the window, still retaining the previous adjustment of the 

 tubes. He will now see two circles, continuing separate as long as he keeps 

 his eyes fixed on the distant surface; and if the finger be held up, as before, 

 in front of one of the tubes, it will appear within the circle which is in front 

 of the other eye, thus causing the impression on the right eye to be appar- 

 ently transposed to the left, and vice versa. 



Exp. 4. Fasten a small disc of white paper on a slip of black pasteboard, 

 of the size suitable for a stereoscope, and place this in the instrament so that 

 the white spot shall be centrally in front of one of the glasses. To a person 

 not aware of the position of the spot, it will appear in the stereoscope as if 

 equally in view to both eyes, and he wall be entirely unable to decide on 

 which retina its picture is impressed. Indeed, properly considered, the spot 

 does not appear directly in front of either eye, but is seen at the intersection 

 of the optic axes, in the medial or binocular direction between the two. 



Let the spot be now moved toward the right side, but still within the range 

 of the left eye, and it will seem to be before the right eye rather than the 

 left. Shift it into the right compartment, but not far from the dividing line, 

 and it will appear as if seen chiefly by the left eye; and, finally, bring tp it 



