HOW COME? Hed here and here and here

How come when I glance at a burning ember, I'll sometimes see a flash of blue? Also, why do I see bright, moving shapes in the blue sky? asks a reader.

Ah, the summer sky. Lazy white clouds, bright sun, the vast, depthless blue, the tadpole-shaped spots. . . . Once you see them, you realize they're everywhere, marring the perfect, serene blue. It's as if the sky were infected with wiggly parasites.

Well, relax. The sky is just fine. It's your eyes that are adding the annoying special effects. When you see bright, moving spots across a blue sky, you're actually watching something moving inside your own body. Crisscrossing the eye's retina are small vessels (capillaries) that carry the oxygen-rich red blood cells that feed our eyes. Besides the red cells, there is also the occasional leukocyte, or white cell, passing through. Cells twist and turn as they flow through the retina's snaking vessels.

The red cells strongly absorb incoming blue light, but they are crammed together, so they don't show up as individual dark spots. Yet we also don't see a distracting road map of dark lines running across our vision. Thankfully, the brain "edits out" the blood-filled capillaries when processing the image.

But white cells don't absorb much blue light. So as the (much bigger) white cells flow by, they appear as moving holes in the capillaries. Presto: We see bright, white shapes in a blue sky.

The blue-sky effect or "Scheerer's Phenomenon" is also known as the blue field entoptic phenomenon. "Entoptic" means "originating in the eyeball," and the paisley sky shapes are just one of many entoptic effects. Another occurs after the sun sets and the blue sky fades to black. We may see it on a summer's night - in the glow of a campfire.

After you are in the dark for one or two minutes and glance at a flame, a burning ember, or a red lightbulb, you may see it: one or two blue arcs of light that flash and fade. (The effect is known as "Purkinje's blue arcs," after the scientist who noticed them in the 1800s.)

According to physicist Jearl Walker, of Cleveland State University, the blue arcs appear because the brain is "misled" by the eye. The eye's receptors, the rods and cones, receive incoming light. The cones are sensitive to colors. When red light strikes the retina, red-sensing cones activate. Because of the red cones' location, some rods that haven't actually received light are also stimulated.

The rods in question happen to be arrayed in an arc. The brain's interpretation: a bright arc. But why blue? Since you've only been in the dark for several minutes, some cones are still sending leftover "yellow light" messages to the brain. But the mistakenly activated "arc of rods" dampens the "yellow" message from yellow-sensing cones in the arc. And when "yellow" signals are inhibited, according to Walker, the brain's response is to perceive "blue." The result: a flashy blue arc, which quickly fades to gray as the eye adapts to darkness.