Russian Spiral (俄羅斯螺旋)

Ai-Hou Wang

With blue lens over right eye and red lens over left eye this will be seen as a stereogram.

The square ring is composed of 12 small squares, a~l.

Going clockwise, square b seems closer to you than square a, c seems closer than b……k seems closer than j, l seems closer than square k and surprisingly, a seems closer than l!! The whole picture looks like step or castle illusion (see picture).

Going clockwise, stepping higher and higher, the red background seems drifting leftward (or the square ring going rightward).

Two eyes diverge gradually, until they can diverge no more.

Going counterclockwise, stepping lower and lower, the red background seems drifting rightward (or the square ring going leftward).

Two eyes converge gradually, until they can converge no more.

Human being has stronger convergence than divergence. The square ring would therefore go leftward farther than rightward.

Intermittent exotropes may have superb divergence range which would push the square ring going rightward more than usual.

You may conversely put the red lens over right eye and the blue lens over left eye.

But above mentioned ‘going forward or backward’, ‘shifting leftward or rightward’ will all be inverse.