Raise a finger to about four inches in front of your nose. Focus on some object in the distance, then -- without changing your focus -- shift your attention back to your finger. Does it seem doubled? Most people claim to be able to experience this, at least after a few tries.
If you then focus carefully on your finger (bringing it out maybe to six or eight inches, depending on how close in you're able to bring your focus), do the objects in the far distance seem unfocused, blurry? Even doubled? Reports of doubling in this case are less common, but I think I can get some doubling in myself in this condition.
One of the great geniuses of the 19th century, a pivotal figure in physics and physiology and psychology, Hermann von Helmholtz writes:
When a person's attention is directed for the first time to the double images in binocular vision, he is usually greatly astonished to think that he had never noticed them before, especially when he reflects that the only objects he has ever seen single were those few that happened at the moment to be about as far from his eyes as the point of fixation. The great majority of objects, comprising all those that were farther or nearer than this point, were all seen double (1910/1962, III.7).
The eminent 18th-century philosopher Thomas Reid writes:
We find that when the eyes are sound and perfect, and the axes of both directed to one point, an object placed in that point is seen single.... Other objects at the same distance from the eyes as that to which their axes are directed do also appear single.... Objects which are much nearer to the eyes, or much more distant from them, than that to which the two eyes are directed, appear double. Thus, if the candle is placed at the distance of ten feet, and I hold my finger at arms-length between my eyes and the candle; when I look at the candle, I see my finger double; and when I look at my finger, I see the candle double: and the same thing happens with regard to all other objects at like distances which fall within the sphere of vision.... You may find a man that can say with good conscience, that he never saw things double all his life; yet this very man, put in the situation above mentioned, with his finger between him and candle, and desired to attend to the appearance of the object which he does not look at, will, upon the first trial, see the candle double, when he looks at his finger; and his finger double, when he looks at the candle. Does he now see otherwise than he saw before? No, surely; but he now attends to what he never attended to before. The same double appearance of an object hath been a thousand times presented to his eye before now; but he did not attend to it; and so it is as little an object of his reflection and memory, as if it had never happened (1764/1997, p. 133-134).
E.B. Titchener, the leading American introspective psychologist circa 1900 (second in eminence only to William James), writes:
The field of vision … shows a good deal of doubling: the tip of the cigar in your mouth splits into two, the edge of the open door wavers into two, the ropes of the swing, the telegraph pole, the stem of another, nearer tree, all are doubled. So long, that is, as the eyes are at rest, only certain objects in the field are seen single; the rest are seen double.... Our habitual disregard ot double images is one of the curiosities of binocular vision (1910, p. 309).
I assume most people now would not agree with such claims.
What's going on? Are these guys, despite their reputations -- and despite Helmholtz's and Titchener's many subtle and interesting introspective discoveries -- simply bad introspectors? I can barely get any doubling, I think, except in the most extreme conditions. I focus on the bookshelf four feet away; the door handle ten feet away seems not at all doubled. Have they unwittingly trained themselves to see double? If so, do they really see most things double, most of the time?
Maybe, I -- we? -- are the bad introspectors? Yet I find it very difficult to imagine that I'm wrong about the singular appearance of that doorknob....