073 THE IRIS DIAPHRAGM

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073 THE IRIS DIAPHRAGM
Iris acts like the diaphragm of a camera,
dilating and constricting the pupil to allow
light of differing intensities into the eye. The
experiment describes how the dilation and
constriction responses of one pupil are geared to
the other.
MATERIAL REUIRED
Card board gt 5cm2 White paper Pin A pair of
scissors Ruler with cm measurements Pencil.
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In the centre of a piece of thin card about 5 cm2
make a pattern of fine pin-holes as shown in
figure. Trim the corners off the card. This is
called a Broca's pupillometer. Close one eye
and hold the card very close to the other,
actually touching your face, and look through the
pin-holes at a brightly lit sheet of plain white
paper.
OBSERVE?
.
The pin-holes appear as a pattern of unfocussed
light discs. Some of the discs appear to
overlap, others will appear to be separated
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Now concentrate on a pair that is nearly touching
but not overlapping. While still concentrating on
this pair of discs with one eye, open the other.
Describe what you observe now. The blurred
images of the pin-holes suddenly become smaller
and sharper. How does this happen? When the
closed eye is suddenly opened, the extra light
entering causes the irises of both eyes to reduce
the size of the pupils and as a result, the
blurred images of the pin-holes become smaller
and sharper. What does this prove? This shows
that the response of one eye is geared to the
other. Did you realize that holding the card
very close to your face actually cuts most of the
light falling on the eye and makes the pupil
dilate? Paul Broca (1824-1880) was a French
surgeon and anthropologist who studied the effect
on behaviour of brain lesions. He located the
area of the brain concerned with speech and thus
was the first person to associate brain regions
with specific functions.
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PINHOLE
CARD
NARROW- PUPIL - WIDE
IRIS
REFRACTION AT LENS
RETINA
SMALLER DISCS THAT SEEM TO MOVE APART PUPILS
CONSTRICT AS A REPONSE TO BRIGHT LIGHT
BLURRED DISCS OF LIGHT USING ONE EYE IN DIM
LIGHT DILATES PUPIL