Blue arcs of the retina

How it works

The retina has a layer of nerve fibres on its inner surface. These fibres carry signals from the light-sensing cells to the optic disc, where they bundle into the optic nerve². Fibres from the centre of vision do not run straight to the disc. They curve above and below the centre in two arcs and meet the disc from the top and bottom².

When a small bright light hits the retina near the centre, the fibres that carry its signal become active. Their activity appears to leak a little into neighbouring cells along the whole path to the disc. The result is a pair of faint glowing arcs that trace the fibre paths¹. They start next to the light and curve towards the blind spot on the same side.

The arcs look blue, or sometimes violet, whatever the colour of the light. The reason for the colour is not settled¹. The arcs appear within a fraction of a second and fade after one or two seconds, even if the light stays on.

Eye doctors have used the arcs as a test. The arcs are weak or missing in eyes with amblyopia, the condition called lazy eye, and in some eyes with early glaucoma. That fits the idea that the arcs come from the nerve fibre layer, which those conditions damage¹.

Try it

1. Sit in a completely dark room. Wait 5 minutes for your eyes to adapt.
2. Use a small red light. The red standby LED on a television or a phone charger works well.
3. Look at a point just to the side of the light, not straight at it. Keep the light within a few degrees of your centre of vision.
4. Two faint blue arcs curve away from the light towards the outer side of that eye.
5. Blink, look away, then look back. The arcs appear again for a second or two.

The arcs are dim. Do not expect them to be obvious the first time.

History and culture

The blue arcs were first described by Jan Evangelista Purkyně in 1825, along with the Purkinje tree and the Purkinje effect¹. They remain one of the least known entoptic effects, because they need full darkness and a very specific light.