Vision Activities
Astigmatism Test Chart:
1.) If you are wearing glasses, remove them. Don’t bother to remove your contacts!
2.) Stand at the line on the floor that is 10 feet from the astigmatism chart.
3.) Cover your right eye with your hand, and look at the chart with your left eye.
4.) Do any of the lines on the chart appear to be much darker or thicker than any of the
others?
Repeat the experiments with your right eye, and also with your glasses on. Any
differences noted?
Snellen Eye Chart: stand at the 20 foot line and read down the chart as far as you can.
Pupil Reflex: (under light stimulus)
1.) Using a millimeter ruler, measure the diameter of the opening of the pupil on your lab
partner.
2.) Do not touch the eye: Measure by holding the ruler close and measuring the distance.
3.) Hold the ruler steady and briefly flash a light in front of the eye.
4.) Quickly measure the change in the diameter of the pupil.
Switch partners and record YOUR results below:
Right eye without light:__________mm with light______________mm
Left eye without light:___________mm with light______________mm
What seems to be happening to the pupil in the presence of bright light?
What structures of the eye are responsible for this event?
Did you “see” anything strange after the flash of light?
Optical Illusions:
Check out some of the way cool illusions I have set up as demonstrations.
Color-Blindness Plates
1.) Examine the colored plates (don’t share your answers with others!)
2.) Record your interpretation of the plates below. (any numbers or symbols seen?)
Color-Blindness is a sex-linked hereditary condition that affects 8% of males, and 0.5%
of females, and is most commonly caused by a lack of Red or Green cones. Please see
your lecture text for additional information about cones.
The Mysterious Purkinje Tree
Normally, our brains suppress the images of the blood vessels of the retina when we
interpret visual information, but we can mess with this a bit.
1.) Gently hold a penlight in your right hand at a 45 degree angle up against the closed
eyelid of your right eye.
2.) This works best in a dimmed light area. (Try the dark room in the back of lab).
3.) Move the penlight very slowly from side to side (only a few millimeters).
If done correctly, this produces a “shadow” of the blood vessels in your retina that you
are able to see!
Were you able to see an image? ________________
Production of an Afterimage
1.) Look at a bright light bulb for a few seconds, then look at a blank wall
*what has happened to you is the production of a “positive afterimage”, which is caused
by the continued production of action potentials by the photoreceptors, even though the
stimulus of bright light has ended. Soon you may see a “negative afterimage”, caused by
the “bleaching” of the photoreceptor pigments, meaning it takes time for the pigments to
regenerate to receive normal stimulus again.