Make a pinhole in a piece of cardboard. Bring your eye close to it and look through the pinhole as you rotate the card. You will see the network of your retinal capillaries against the background of a cloudy sky. How does this happen?
鈥� This is a fascinating phenomenon known as Purkinje shadows, after the Czech physiologist and neuroanatomist . It also illustrates an excellent argument against intelligent design.
In the human eye, light passes through all the nerve fibres and blood vessels before reaching the photoreceptors. This curious arrangement means that the blood vessels cast shadows, and it explains why the capillaries can be seen if you look through a moving pinhole. Surely a master creator wouldn鈥檛 have made a mistake like that. After all, the squid eye is designed the other way around, which raises the possibility that the mythical intelligent designer of life considers cephalopods a higher form of life than humans.
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The reason we don鈥檛 normally see the shadows of the blood vessel is because the human eye is incapable of registering a stationary image. We can see things that don鈥檛 move, such as statues or doors, only because our eyes are continually making tiny movements which ensure that their image jiggles across our retina. Because the blood vessels are part of the eye they move with it and usually remain invisible. Using sophisticated eye-tracking equipment it is possible to completely stabilise a retinal image. When this happens the image disappears, a phenomenon called Troxler鈥檚 fading. If our eyes were completely still we鈥檇 be almost blind. Intelligent design, huh?
Moving a pinhole across the pupil changes the direction of light reaching the back of the eye, which has the effect of 鈥渕oving鈥� the capillaries relative to the retina, making them visible. An even better way to see the blood vessels in your own eye is to put a small bright pen torch near the white part of the eye (while being careful not to poke yourself).
Ian Flitcroft, Consultant Ophthalmologist, Dublin, Ireland