See inside your eye Part 1 Using a point light source to see floaters and cataracts
See inside your eye with that same eye. Part1. Here is a list of six techniques scientistshave described for seeing inside your eye with that same eye. The oldest technique, which is the topic ofthis tutorial, involves using a point light source to see clear shadows of floaters in your eyeand any spots you might have in the lens of your eye. In this tutorial I will demonstratea number of ways to do this while reviewing the history and explaining some of the science.I will also show numerous examples of what you might see in your own eyes.
The other techniques allow you to see bloodmoving in capillaries on the retina, shadows of the blood vessels on the retina, nerveactivity on the retina, and the back of the eye. I hope to cover these other techniquesin future tutorials. But let's get back to the topic of this tutorial,the first and oldest technique for seeing inside your own eye with that same eye, startingwith a little science history. The German astronomer Johannes Kepler publisheda book on optics in 1604. in which he stated that our eyes function by focusing an imageonto the back of the eye. The physicist and astronomer Christoph Scheiner proved Keplerwas right by removing most of the tissue from
the back of actual eyes and looking at thefocused images. Then, in 1674, the French mathematician ClaudeFran ois Dechales described how a near sighted person or someone holding a magnifying lensin front of their eye can see the shadows of defects in the eye in the unfocused imageof a distant light source. It was later discovered that looking at a brightly lit surface througha pinhole in a card held close to the eye served the same purpose, as well as otherlens arrangements and looking closely at the reflection of a distant light in a tiny dropof mercury on black felt. Let me explain how these various methods workand also demonstrate these and some other
variations. Light rays from a sufficiently distant andsmall source arrive at the eye almost parallel. The light can be a candle like Dechales used,or something like a AAA mini Maglite(R) in candle mode which I prefer to use. The corneaand lens of the eye refract this light to focus it to a point on the retina. But ina nearsighted person this light will be focused to a point in front of the retina then itwill spread out on the retina. It will appear as a disk because its shape is determinedby the shape of the opening in the iris, which is round in humans. Holding a magnifying lensin front of your eye will do the same thing.
Not only does the border of the iris casta shadow, but so does whatever might be in the path of the light, like a dark spot inthe lens of the eye. You can also use a concave lens, perhaps onesalvaged from the viewfinder of a junk camera, to create an unfocussed image of a distantlight on your retina. Or you can cancel out the focusing effect of the cornea by fillinga swim goggle with saline solution. Finally, you can use a tiny, shiny, convex surface,like a small drop of mercury on black felt or the tines of a fork held to reflect light.What you are actually doing is creating a virtual image of the distant light. This imageis functionally the same as a tiny light source
held too close to your eye for you to focuson. You can also create a real image using a magnifying lens. However, using a mirroror lens has the disadvantage that you will see any imperfections in the mirror or lensas shadows, along with the shadows of things on or in your eye. The best thing to do is to simply create apoint source of light that can be held close to your eye. One of the easiest ways to dothis is to crumple up some aluminum foil, then flatten it out. Put this over a flashlightand you will notice numerous tiny holes. Put a piece of matte finish cellophane tape oversome of the holes to diffuse the light, then