Have you ever noticed something swimming in your field of vision? It may look like a tiny worm or a transparent blob, and whenever you try to get a closer look, it disappears, only to reappear as soon as you shift your glance. But don't go rinsing out your eyes! What you are seeing is a common phenomenon known as a floater. The scientific name for these objects is Muscae volitantes, Latin for "flying flies," and true to their name, they can be somewhat annoying. But they're not actually bugs or any kind of external objects at all. Rather, they exist inside your eyeball. Floaters may seem to be alive, since they move and change shape, but they are not alive. Floaters are tiny objects that cast shadows on the retina, the light-sensitive tissue at the back of your eye. They might be bits of tissue, red blood cells, or clumps of protein. And because they're suspended within the vitreous humor, the gel-like liquid that fills the inside of your eye, floaters drift along with your eye movements, and seem to bounce a little when your eye stops. Floaters may be only barely distinguishable most of the time. They become more visible the closer they are to the retina, just as holding your hand closer to a table with an overhead light will result in a more sharply defined shadow. And floaters are particularly noticeable when you are looking at a uniform bright surface, like a blank computer screen, snow, or a clear sky, where the consistency of the background makes them easier to distinguish. The brighter the light is, the more your pupil contracts. This has an effect similar to replacing a large diffuse light fixture with a single overhead light bulb, which also makes the shadow appear clearer. There is another visual phenomenon that looks similar to floaters but is in fact unrelated. If you've seen tiny dots of light darting about when looking at a bright blue sky, you've experienced what is known as the blue field entoptic phenomenon. In some ways, this is the opposite of seeing floaters. Here, you are not seeing shadows but little moving windows letting light through to your retina. The windows are actually caused by white blood cells moving through the capillaries along your retina's surface. These leukocytes can be so large that they nearly fill a capillary causing a plasma space to open up in front of them. Because the space and the white blood cells are both more transparent to blue light than the red blood cells normally present in capillaries, we see a moving dot of light wherever this happens, following the paths of your capillaries and moving in time with your pulse. Under ideal viewing conditions, you might even see what looks like a dark tail following the dot. This is the red blood cells that have bunched up behind the leukocyte. Some science museums have an exhibit which consists of a screen of blue light, allowing you to see these blue sky sprites much more clearly than you normally would. While everybody's eyes experience these sort of effects, the number and type vary greatly. In the case of floaters, they often go unnoticed, as our brain learns to ignore them. However, abnormally numerous or large floaters that interfere with vision may be a sign of a more serious condition, requiring immediate medical treatment. But the majority of the time entoptic phenomena, such as floaters and blue sky sprites, are just a gentle reminder that what we think we see depends just as much on our biology and minds as it does on the external world.
Da li ste ikada primetili da nešto pliva u vašem vidnom polju? Može izgledati kao crvić ili providna grudvica, a kada god pokušate da ga izbliza pogledate, nestane, da bi se opet pojavilo čim skrenete pogled. Nemojte odmah ići na ispiranje očiju! Ono što vidite je uobičajena pojava poznata kao „mušice”. Naučno ime za ove objekte je Muscae volitantes, latinski za „leteće mušice”, a u skladu sa svojim imenom, mogu biti i malo dosadne. Međutim, to uopšte nisu bube ili neka vrsta spoljašnjih objekata, već postoje unutar vaše očne jabučice. Mušice mogu da deluju kao žive jer se kreću i menjaju oblik, ali nisu. Mušice su sićučni objekti koji bacaju senku na mrežnjaču, tkivo osetljivo na svetlost u zadnjem delu našeg oka. Mogu biti delovi tkiva, crvena krvna zrnca, ili grudvice proteina. Pošto vise u staklastom telu, želatinastoj supstanci koja ispunjava unutrašnjost našeg oka, mušice plove uz pokrete vašeg oka i čini se da pomalo skakuću kada se oko zaustavi. Mušice se jedva mogu primetiti veći deo vremena. Postaju vidljive kada su bliže mrežnjači, kao što kada približavanje ruke stolu sa svetlom iznad njega ima za rezultat jasnije definisanu senku. Mušice su posebno primetne kada gledate ujednačenu sjajnu površinu, kao što je prazan kompjuterski ekran, sneg ili jasno nebo, kada konzistencija pozadine čini da se lakše primete. Što je svetlost jača, vaša zenica se više skuplja. Ovaj efekat je sličan onome kada na mesto jakog difuznog svetla postavite samo jednu sijalicu, što takođe čini da se senka jasnije pojavljuje. Postoji još jedna vizuelna pojava koja izgleda slično mušicama, ali nije zapravo povezana sa njima. Ako ste videli tačkice svetla kako okolo lutaju dok gledate u jasno plavo nebo, doživeli ste ono što je poznato kao entoptički fenomen plavog polja. Na neki način, ovo je suprotno od primećivanja mušica. Ovde ne vidite senke, već male pokretne prozore koji propuštaju svetlo u vašu mrežnjaču. Prozore zapravo stvaraju bela krvna zrnca koja se kreću kroz kapilare pored površine mrežnjače. Ovi leukociti mogu biti toliko veliki da skoro ispunjavaju kapilar, uzrokujući da se prostor sa plazmom otvori ispred njih. Pošto su i prostor i bela krvna zrnca vidljiviji na plavom svetlu nego crvena krvna zrnca koja se obično nalaze u kapilarima, vidimo pokretnu tačkicu svetla gde god da se ovo dešava, dok prati puteve vaših kapilara i kreće se u skladu sa vašim pulsom. Pod idealnim posmatračkim uslovima, možda ćete čak videti i nešto što izgleda kao tamni rep koji prati tačku. Ovo su crvena krvna zrnca koja su se grupisala iza leukocita. Neki muzeji nauke imaju izložbu koja se sastoji od ekrana plavog svetla, omogućavajući vam da vidite ove duhove plavog neba mnogo jasnije nego obično. Dok svačije oči doživljavaju ovakvu vrstu efekata, broj i tip se znatno razlikuju. U slučaju mušica, često se provuku neprimećeni, pošto su naši mozgovi naučili da ih ignorišu. Međutim, neuobičajeno brojne ili velike mušice koje ometaju vid mogu biti znak ozbiljnijeg stanja, što zahteva momentalno medicinsko lečenje, ali u većem broju slučajeva, entoptičke pojave kao što su mušice ili duhovi plavog neba, samo su blagi podsetnik da ono što mislimo da vidimo zavisi onoliko od naše biologije i umova