The Spots Before Your Eyes
On one of the last few beautiful days of summer, I was lying back in our lawn chair looking at a clear, blue Alaska sky. Actually, what I was doing was not so much looking at the sky, as watching those pesky spots, blobs and ropey filaments wandering around seemingly before my eyes.
My experience, which is probably not much different than anyone else's, is that when I stare at a featureless background such as a clear sky, my field of view contains a number of specks or more complicated forms which remind me of nothing so much as pictures I've seen of bacteria. Sometimes the "bacteria" are straight, and at other times it looks as if they've been tangled up or dumped into a heap. Although some forms are surprisingly clear, I can never quite focus on them, and if I turn my eyes in order to concentrate on one, it outraces me outside my field of vision.
Observing these questionable objects can be an amusing and harmless pastime if one has nothing else to do, but at times they can be an absolute nuisance and even interfere with reading, as they were doing with me on that afternoon.
Although I'd wanted to know more about this illusion for years, partially to assure myself that I wasn't a freak of some kind, I finally decided to see if I could find some literature on the matter. Fortunately, a reference turned up almost immediately in the form of an article by Jearl Walker in the April, 1982, issue of Scientific American.
Walker, citing earlier researchers, calls the visual phenomena "floaters." Floaters are old red blood cells that are suspended in the fluid at the rear of the eye just in front of the rear wall, or retina. The retina is that portion of the eye where images are focused, and it functions something like a movie screen.
The figures seen are not actual images of the blood cells, but are reasonably good approximations. What is actually seen is the pattern made when light rays are diffracted around a floater and recorded on the retina.
The red blood cells comprising floaters are more or less derelicts in the fluid of the eye, having leaked out of the retina. Once they are free, they swell from their normal platelet shape into spheres, and in so doing, lose their hemoglobin and thus their red color. Although they appear much larger to the observer, a swollen cell is only about one-fiftieth the diameter of the period at the end of this sentence. They can appear singly, or be strung together like beads on a string.
The closer a floater is to the retina, the clearer the image, and the smaller it appears. This is because the diffraction patterns formed on the retina are better focused at short distances. Images formed by more distant cells may be too fuzzy and diffuse to be distinguished. Therefore, if you want to spend some time contemplating strings of your old red blood cells, you would do best to lie on your back. This way, floaters will gradually settle to the back of the eye near the retina.
It would seem reasonable that in order to "catch up" with a floater that is drifting out of the scene, it would be best to turn the eye quickly toward it, in the belief that the motion of the fluid in the eye would take a moment to follow. In actuality, the reverse is true. If you wish to bring a floater back to front stage, it is best to turn your eyes from the direction in which it is drifting. This is because the brain reverses the image that falls on the retina before we perceive it. In other words, the floater that appears to be crossing from left to right is actually crossing from right to left.
One other thing that occurred to me on that sunny afternoon on the lawn was that I seem to observe more floaters as I get older. Sadly, this seems to be the case with most people. Retinas, it seems, grow more leaky with age.
