Experiment 1 – The Blind Spot
X
1. Mark an X close to the left edge of a piece of paper (see above, printed or drawn by hand is OK). Hold the paper
in front of you at about 450 mm (or 45 cm) from your eye. Record this distance. Call this d. The X should be
directly in front of your right eye with the rest of the blank page off to the right.
2. Close your left eye and look at the X with your right eye. Hold the tip of a pen/pencil just in front of the X on the
paper so that the pen covers the X.
3. Slowly move the pen across the page to the right away from the X while still looking directly at the X, viewing
the pen with only your peripheral vision.
4. Mark the paper at the position where the tip of the pen disappears (it will be about 150 mm (15 cm) to the right of
the X when the paper is at about 450 mm (45 cm) distant from your face). At this point the image of the pen tip has
fallen on the blind spot of your right eye. Record the distance from the X to the mark. Call this l.
5. Once you have located the center of the blind spot, keep the pen at the blind spot location and while continuing to
stare at the X, very carefully move the pen both left and right and up and down so you can determine the
approximate size of the blind spot (the blind spot is close to circular). Measure and record this approximate size
(diameter) of the blind spot on the paper in mm or cm. This diameter will be quite small compared to the distance
from the X.
6. Turn the paper upside down so the X is now on the right edge of the paper, close your right eye, and repeat steps 1
to 4 for your left eye, moving pen to the left until it disappears to locate the blind spot and its size for your left eye.
Questions after doing the experiment.
1. Are your two eyes bilaterally symmetric with respect to the left and right sides of your head?
Bilateral symmetry is left/right symmetry on either side of a vertical plane. (0.5 points)
2. Considering the anatomy of either eye individually, is a single eye bilaterally symmetric with
respect to the left and right halves of that eye? (0.5 points)
3. Why aren’t you normally aware of the blind spots? (0.5 points)
4. About how far off-axis is the blind spot in degrees? See the Figure 2 on the last page for how to
calculate this angle q. (0.5 points)
5. What is the diameter of the blind spot in degrees? Again, calculate q (see below). (0.5 points)
6. Is the blind spot located on the part of your retina nearer to your nose (“nasal”), or nearer to your
ear (“temporal”)? How did you deduce your answer? (0.5 points)
