# No Spirit Level: Can you level a beam by eye?

In a dark-room subjective visual horizontal test, 95% of 72 healthy participants placed the line within ±2.5° of true horizontal. Five attempts across distinct visual challenges measure whether your balance drifts left or right.

## The Testing Mechanism

On a virtual workshop bench, an aluminum beam rests on a fulcrum. A steel ball bearing is placed at the center. The user adjusts the beam angle until it appears perfectly level (0.00°), then releases the ball.

If the angle deviates from 0.00° by more than ±0.15°, gravitational acceleration ($a = g \sin\theta$) sends the ball rolling toward the lower stop. A hidden digital level immediately reveals the exact error in degrees.

## The 5 Rounds

1. **Beam on clean surface**: Baseline measurement of subjective visual horizontal.
2. **Off-center fulcrum**: The fulcrum is shifted left; the longer right arm induces a false visual sense of droop.
3. **Background lines tilted at 12°**: The hatched backdrop triggers the Zöllner / Rod-and-Frame illusion, shearing perceived orientation in the visual cortex.
4. **Short, heavy bar**: A compact aspect ratio reduces the visible displacement at the ends.
5. **Long workshop beam**: Maximum precision test across a wide span.

## Why does the brain misjudge the horizontal?

The horizontal plane is estimated by combining vestibular gravity inputs from inner ear otoliths with orientation hypercolumns in primary visual cortex (V1). Asymmetric spatial frames or tilted context lines warp angular judgment by several degrees.

All measurements are computed locally in the browser.

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Bilingual edition published by Marius Comper: https://mariuscomper.uk/fara-cumpana/en/