| Advance preparation | | | | moving. |
| Cut discs about 6 cm in diameter from thin card and | | | | (e) There are no capillaries over the fovea. Can you |
| make a pin-hole in the centre of each. The hole made | | | | pick out this area in the image you are forming? |
| by the spike on a pair of compasses or dividers is | | | | Discussion |
| about the right size. Provide each group with a disc, a | | | | The capillaries lie in front of the sensory cells of the |
| sheet of white paper and, if the lighting in the room | | | | retina and cast a constant shadow on these cells. |
| is not very good, a bench lamp or some source of | | | | Since stationary images are suppressed or not |
| illumination. | | | | transmitted, however, we are not normally aware of |
| Experiment | | | | this capillary network. Moving the pin-hole around |
| (a) Close one eye and hold the card close to the | | | | constantly changes the light intensity on the retina |
| other. | | | | and the retinal cells are temporarily stimulated by the |
| (b) Look through the pin-hole at a brightly lit sheet of | | | | shadow of the capillaries. |
| plain white paper about 30 to 40 cm away. | | | | This effect should not be confused with the |
| (c) Move the card about very slightly with a circular | | | | impressions of moving filamentous objects seen |
| motion so that you can see through thepin-hole all | | | | when the eyes are half closed when viewing a bright |
| the time. | | | | area such as a clear sky or white surface. The |
| (d) Allow your eyes to relax and a net-like pattern of | | | | 'floating' images seen in these conditions are usually |
| capillaries will appear against the whitebackground | | | | attributed to debris suspended in the vitreous |
| after a few seconds provided you keep the card | | | | humour. |