Compass in your eye

PEOPLE CAN see the Earth's magnetic field, albeit unconsciously.
The effect is too small to be noticeable, but other animals may use
their eyes in this way to get their bearings, says a report in New
Scientist.

The Earth's magnetic field lines vary depending where you are. They
always run south to north, but are horizontal only at the equator,
dipping in at a steeper angle the greater the latitude.

Birds and some other migrating animals seem to use these angles to
help them navigate, and one theory is that they do so using lightsensitive
cells.

German researchers investigated whether people's eyes are sensitive
to these field lines .

They measured the lowest level of light that people could detect in a
small spot straight ahead of them. They had people face south, west
and south south-west, and used a magnetic coil to create a horizontal
north-facing field.

They then repeated the experiment with the Earth's natural magnetic
field, which in Germany is angled 70 degrees downwards towards
the north.

When the field lines coincided with the direction of the spot, which
only occurred when people faced south in a horizontal field, the
threshold of brightness at which their eyes first detected a very dim
light went up.

The effect was small but significant, say the researchers. Although
we are not conscious of it, the same probably happens when
photoreceptors in the eye are aligned with the Earth's magnetic field.
American researchers studying newts say the light compass in them
probably use specialized photoreceptors to detect the magnetic field
lines.

Magnetic fields interact with spinning electrons, and could
theoretically influence photoreceptor chemicals at the quantum level,
altering their efficiency.