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Advance preparation

Cut discs about 6 cm in diameter from thin card and make a pin-hole in the centre of each. The hole made by the spike on a pair of compasses or dividers is about the right size. Provide each group with a disc, a sheet of white paper and, if the lighting in the room is not very good, a bench lamp or some source of illumination.

Experiment

(a) Close one eye and hold the card close to the other.

(b) Look through the pin-hole at a brightly lit sheet of plain white paper about 30 to 40 cm away.

(c) Move the card about very slightly with a circular motion so that you can see through the

pin-hole all the time.

(d) Allow your eyes to relax and a net-like pattern of capillaries will appear against the white

background after a few seconds provided you keep the card moving.

(e) There are no capillaries over the fovea. Can you pick out this area in the image you are forming?

Discussion

The capillaries lie in front of the sensory cells of the retina and cast a constant shadow on these cells. Since stationary images are suppressed or not transmitted, however, we are not normally aware of this capillary network. Moving the pin-hole around constantly changes the light intensity on the retina and the retinal cells are temporarily stimulated by the shadow of the capillaries.

This effect should not be confused with the impressions of moving filamentous objects seen when the eyes are half closed when viewing a bright area such as a clear sky or white surface. The ‘floating’ images seen in these conditions are usually attributed to debris suspended in the vitreous humour.

 
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