Human senses - seeing

1. Human senses - seeing

2. Optic disc (blind spot) evidence A Hold picture in stretched arm in front of your eyes. Close your left eye and look at the left “A” at the rectangle with your right eye. Move the picture towards your eyes slowly and observe what happens. Conclusion: During the observation of the picture right “A” disappears (Rays fall to the retina and some cells inform us about it; rays from the area, where the right “A” is situated, fall to the part of retina in certain moment, where there are no sensory cells because fibers of the optic nerve come out from the eyeball).

3. Optic disc (blind spot) evidence B Hold picture in stretched arm in front of your eyes. Close your left eye and look at the cross at the rectangle with your right eye. Move the picture towards your eyes slowly and observe what happens. Conclusion: During the observation of the picture white area is covered with black and the line seems to be continuous. (Rays from the area, where white part is situated, fall to the optic disc, and the brain completes the picture as consider it to be logical).

5. Black and white vision (negative afterimages) During 30 seconds gaze at the black cross in the disc. After this time look intently at the black cross next to the disc wait for a moment and observe what happens. Conclusion: During the observation of the black cross we can see afterimage of the same shape as the left disc but in reversible colours. (The rod cells in retina give us black and white vision, and our night vision. They use a chemical called rhodopsin = fotosensitive pigment. When a photon of light hits a molecule of rhodopsin, the molecule changes and sends a signal to our brain. During this experiment rod cells are overexcited - especially cells, which were hit by big amount of photons coming from the white parts of the disc /the picture is “burned out” in the retina for a short time/. When we look at the cross on a white paper, this rod cells are still exhausted, thus this parts seem to be dark, on the other hand rested cells, which interact with black parts, see the white colour very intensively. This sensation lasts until the pigment is not recovered.)

6. Spatial vision Hold two pencils in stretched arms with sharp tips turned towards each other at the distance about 40 cm between. Move them closer and closer and try to touch one tip to another. Try it with both eyes opened several times and then try the same with one eye closed. You can alternate the eye. Record the results and compare them. Conclusion: You are less successful when one eye is closed. (The spatial vision, which develops by covering of visual fields of both eyes, is necessary for good and precise seeing.) Complement 1: Look at the stretched finger in front of your face and watch it alternately by right or left eye. (Seeming movement of the finger is the result of visual fields alternating.) Complement 2: Try to compare spatial vision of different animals and try to find some connections between range of spatial vision and e.g. some ethological or ecological characteristics etc.

7. Eye lens accommodationSit against the window and hold the pencil in the distance of 30 cm in front of your eyes. Try to observe the pencil and some faraway object. Compare the vision sharpness. Conclusion: It is not possible to see both objects sharply in a same moment. (Close seeing needs strong lens curvature, far seeing conversely.) Complement: Hold the pencil in a same way as mentioned above and move it closer to your eyes slowly. (We feel pain in our eyes in certain distance because of too high tension of lens curvature muscles.)