Most science experiments for adults need a kit, a garage, or at least a trip to the store. This one needs a clear blue sky and about two minutes. It lets you watch your own white blood cells moving through the blood vessels at the back of your eye, live, using nothing but the eye itself.

It’s the kind of experiment the 25Pages STEM series is built around: make a prediction, test it with your own observation, then understand the science well enough to explain it to someone else.

Make a prediction first

Before you look, write down your answers to three questions. If tiny bright dots appear, will they move randomly or in a rhythm? Will they follow your eyes when you look around, or move on their own? Will you see them better against blue sky or a white wall?

What you need

• A clear, bright blue sky. Face away from the sun.
• Optional: a blank, evenly lit blue screen on a computer or tablet for a cloudy day
• A watch, or two fingers on your wrist to feel your pulse
• A notebook to record what you see

Step-by-step

1. Turn your back to the sun. Look at an open patch of blue sky well away from it.
2. Relax your focus. Don’t focus on anything in particular. Let your gaze rest on the blue.
3. Watch for tiny bright specks. Within a few seconds, most people notice small points of light darting along short, curved paths. Some seem to leave a faint dark tail.
4. Check the rhythm. Place two fingers on your wrist. Do the specks speed up in time with your heartbeat?
5. Move your eyes, then hold still. Notice whether the dots stay put relative to your vision or drift like floaters.
6. Compare backgrounds. Try a white wall or a gray sky. Record where the dots are easiest to see.

What you are actually seeing

Those darting specks are white blood cells flowing through the tiny blood vessels in front of your retina, according to the American Academy of Ophthalmology. Red blood cells, which make up more than 90 percent of your blood, absorb blue light. White blood cells let blue light pass through to the retina, which reports a brief spot of extra brightness to the brain.

White blood cells are nearly as wide as the smallest capillaries. Red blood cells pile up behind them, leaving a clear gap of plasma in front. Light shining through that gap is what you see as a fast-moving white dot, Smithsonian Magazine explains. That’s also why some dots seem to have a dark tail: the bunched-up red cells following behind.

Eye doctors call this the blue field entoptic phenomenon. A clinical paper in the Indian Journal of Ophthalmology describes the particles moving in pulses synchronized with the heartbeat. Under ideal viewing conditions, it reports, about 150 to 200 particles can be seen. Clinicians have used special blue-light instruments based on this effect to help assess the eye.

How it’s different from floaters

Floaters are clumps in the gel that fills your eye. They vary in size and drift slowly after your eye moves. Blue-sky dots are all the same size and shape, and they keep racing along even when your eye is still, the AAO notes. Telling the two apart is a small exercise in careful observation, the same skill every scientist relies on.

Turn the observation into an experiment

• Heart rate test: observe at rest, then again after a minute of brisk stair-climbing. Do the dots move faster?
• Background test: rate how visible the dots are (0 to 5) against blue sky, a white wall, and a blue screen.
• Time-of-day test: repeat in the morning and afternoon on clear days, and record any differences.
• Compare observers: ask a friend or family member to try it. Not everyone notices the dots, and comparing notes is part of the science.

Safety notes

• Never look at or near the sun, even briefly, and never use binoculars, cameras, or lenses for this experiment.
• Keep sessions short. If your eyes feel strained, look away and rest.
• This is an observation activity, not a medical test. If you notice a sudden increase in floaters, flashes of light, or a shadow over part of your vision, contact an eye doctor promptly.

Check your prediction

Go back to your notes. Did the dots pulse with your heartbeat? Did they follow your gaze? Explaining why in your own words is where a two-minute observation becomes lasting knowledge. That’s the core idea behind vertical learning: master one idea deeply enough to teach it.

Keep learning, one idea at a time

If experiments like this keep you curious, The Chronicle MMXXVI Editor’s Edition is the 25Pages annual hardback for lifelong learners and self-directed adults. The DIY Blue Book of Experiments MMXXVI Editor’s Edition collects hands-on experiments that connect science to everyday life, from the chemistry of cooking to strategic thinking. Explore the full collection at 25Pages.com.