Purkinje tree

How it works

The blood vessels of the retina lie in front of the light-sensing cells, not behind them. Every image that reaches the rods and cones passes through a net of vessels first. The vessels cast a shadow on the cells below¹.

You never see this shadow in normal life. It never moves relative to the retina, and the visual system stops reporting anything that stays perfectly still. This is the same adaptation that causes Troxler fading³. The cells under the vessels are simply treated as the baseline.

If light enters the eye from an unusual direction, the shadow lands on different cells. For a moment it is a new signal, and you see it: a dark branching tree on a bright field. The trunk sits on the side where the optic nerve leaves the eye, and the branches spread across the retina¹. The tree is missing from the centre, because the fovea has no vessels on top of it.

The image only lasts while the shadow moves. Hold the light still and the tree fades within a second or two. Move the light and it reappears¹.

Try it

1. Go into a dark room. Wait a minute for your eyes to adapt.
2. Take a small torch or a phone light. Close one eye and open the other.
3. Hold the light close to the outer corner of the open eye, pointing in across the white of the eye. Do not shine it straight into the pupil.
4. Wiggle the light gently.
5. A dark branching pattern appears against a glow. Those are the blood vessels of that eye.
6. Stop moving the light. The tree fades. Move it again and it returns.

An eye doctor’s slit lamp produces the same figure during an exam.

History and culture

The figure is named after Jan Evangelista Purkyně, a Czech physiologist. He described it in 1823 after noticing the pattern while looking at glowing embers². He also discovered the Purkinje effect and the Purkinje images, both of which have entries on this site.