This shift is even more dramatic when you hold an index finger about 18 inches in front of your nose and perform the above operation. This apparition is instigated by the principle of parallax. Your line of sight from each eye is not quite parallel to the selected target, but converges from the eye pair, so that the left eye sees a bit of the left side of the target not seen by the right eye while that right picks up a bit of the right missed by the left. If you have access to a pocket stereoscope, you can most likely see the stereo effect by placing it, with legs extended, against the image below as it appears on the screen. If you do not have a stereoscope, you still may be able to get the 3-D effect without one.
Many people can see in stereo with the unaided eye.
To test the likelihood of doing this, contact your two index fingers tip to tip at eye level about 12 inches in front of your nose. Moving the fingers closer to or farther from the eyes causes the sausage to expand or shrink. Try this natural stereo viewing either on the image pair above or the one below. You may need to move your eyes in or out from the screen as well. If you are predisposed to this viewing, the two images will seem to fuse into a third image between them that has the desired 3-D effect.