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Biology LibreTexts

Part 2: Sight

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  • 2A: Blind spot test

    The optic disk, the sight where ganglion cell axons exit the eye, does not contain photoreceptors. We do not perceive the blind spot because the brain interpolates information to fill in the gaps. You can locate the blind spot by moving the image below toward your head.

    Sight 1.PNG

    Figure \(\PageIndex{1}\) Standard blind spot test

    Directions: With your right eye (left closed), stare at the cross, and move the paper towards your eyes until the circle disappears. Measure the distance from the image to your eye. You can repeat this with your left eye, staring at the circle. The cross will disappear at a distance related to the physical separation of your fovea and your optic disk. You can estimate this distance (d’, in mm) of your retinal structures by recording the distance when the image disappears (x), and measuring the distance between the cross and circle in the image (d). This is actually a ‘reduced eye’ model, which involves some approximations.

    Sight 2.PNGd’ = 17 (d/x)

    d’ = fovea to optic disk (mm)

    d = cross to circle in image (mm)

    x = eye to image (mm)

    Figure \(\PageIndex{2}\) Diagram to show measurement of blind spot to fovea


    1. What is the distance between the fovea and optic disk in your left eye?
    2. Right eye?
    3. Average distance?
    4. Are your eyes exactly the same?
    5. Hypothesize why or why not.

    2B: Color-blindness test

    Color-blindness can be tested with appropriate Standard Pseudoisochromatic plates (e.g. Ichikawa et al., ISBN 0-89640-030-1)


    Using the plates are you able to detect the image present for the presented colors? Make a table of the image that you see and the corresponding plate number.

    If you exhibit some degree of colorblindness, what type? What may be happening with your cone cells within your retinas? Read back a little to see if you can figure it out.

    2C: Visual acuity test

    Visual acuity refers to the sharpness or clarity of vision, and is an indication of the focusing capacities of your eyes, especially the lens and cornea.


    Use the standard eye chart (Snellen chart), at the appropriate distances (usually 20 feet), to measure your acuity. Comparisons on the chart will refer to this standard measure. To use the chart, find the red tape on the floor. Stand here and cover one eye without squinting. Have your partner stand near the chart and tell you which line to read starting from the top. When you can no longer read a line accurately with one eye, then this is the acuity for that eye. Write it down. Now do the other eye the same way. Do you have the same acuity in both eyes? Why or why not?

    Note on corrective lenses: If you have contacts, keep them in, but if you wear glasses you may want to try the test with and without them to see just how much your corrective lenses improve your vision!