eye activities by HC120929152753

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                                         Special Senses: The Eye

Station 1: Eye Model
 View the model of the eye. Draw a sketch of the model below and label the accessory structures of the
eye.




Observe the eyes of another student and identify as many accessory structures as possible. Ask the
student to look to the left. Which eye muscles produce this action?

Right Eye _______________________             Left Eye _______________________

Ask your partner to look superiorly. What muscles bring about this motion?

Right Eye _______________________             Left Eye ________________________

Station 2: Visual Tests
Test 1:
Hold the figure below about 18 inches from your eyes. Close your left eye, and focus your right eye on
the X, which should be positioned so that it is directly in line with your right eye. Move the figure
slowly towards your face, keeping your right eye focused on the X. When the dot focuses on the blind
spot, which lacks photoreceptors, it will disappear!

Record in centimeters the distance at which this occurs. The dot will reappear as the figure is moved
closer. Distance at which the dot disappears. Right Eye _________.

Repeat the test for your left eye. This time close your right eye and focus your left eye on the dot.
Record the distance at which the X disappears. Left Eye ________________.
Test 2:
Lens elasticity can be tested by measuring the near point of accommodation. The near point of vision is
about 10cm for young adults. (Closer in children and farther in old age). To determine your near point of
accommodation, hold a common straight pin at arm’s length in front of one eye. Slowly move the pin
toward that eye until the pin image becomes distorted. At this point, have your partner measure the
distance from your eye to the pin (in cm) and record the distance below. Repeat this procedure for the
other eye.

Near point for right eye ___________________          Left eye _____________________

Station 3: Snellen’s Eye Chart
Visual acuity (sharpness of vision) is generally tested with Snellen’s Eye Chart, which consists of letters
of various sizes printed on a white card. The distance at which the normal eye can read a line of letters is
printed at the end of that line.

   1. Have your partner stand 20 feet from the posted Snellen’s eye chart and cover one eye with their
      hand. As your partner reads each consecutive line aloud, check for accuracy. If this individual
      wears glasses, give the test twice – first with glasses off and then with glasses on.
   2. Record the number of the line with the smallest-sized letters read. If it is 20/20, the person’s
      vision for that eye is normal. If it is 20/40, or any ratio with a value less than one, he or she has
      less than normal visual acuity. (Such an individual is myopic). If the visual acuity ratio is greater
      than 1, vision is better than normal.
   3. Repeat the process for the other eye

       Visual acuity right eye _________________ Left eye __________________

Station 4: Color Blindness
Ishihara’s color-blindness plates are designed to test for deficiencies in the cones or color photoreceptor
cells. There are 3 cone types – one that primarily absorbs blue wavelengths, another absorbs mostly
green wavelengths, and a third absorbs primarily red light. Nerve impulses reaching the brain from these
different photoreceptor types are then interpreted (seen) as blue, green, and red, respectively.
Interpretation of the intermediate colors of the light spectrum is the result of simultaneous input from
more than one cone type.

   1. View the color plates in bright light or sunlight while holding them about 30 inches away and at
      right angles to your line of vision. Report to your lab partner what you see in each plate. Take no
      more than 3 seconds for each decision.

   2. Your partner is to write down your responses and then check their accuracy with the correct
      answers provided in the color plate book. Is there any indication that you have some degree of
      color blindness?

       ______________ If so, what type? ____________________________________
Station 5: Optical illusions

View the various optical illusions at this station. What do they tell you about your perception?




Station 6: Testing for Astigmatism

The astigmatism chart tests for defects in the refracting surface of the lens and/or cornea. View the chart
below first with one eye and then with the other, focusing on the center of the chart. If all the radiating
lines appear equally dark and distinct, your refracting surfaces are not distorted. If some of the lines are
blurred, or appear less dark than others, you have at least some degree of astigmatism. Is astigmatism
present in your right eye? __________ Left eye? ___________




Station 7: Depth Perception Test

Set up apparatus at edge of desk with white background card inserted, and place chair 2.5m away to
start. At the beginning of each trial, have test subject let go of the strings and look away from the arrows
as partner adjusts the pointers to a misaligned position. The subject sits in the chair, keeps the pointers
at eye level, and tries to line up the arrows by moving the two ends of the attached string.

Test the ability of each subject to align the arrows with only the left eye open, with only the right eye
open, and with both eyes open. Record the distance between the two arrows after each trial. Repeat with
black background card.


            Right       Left       Both
             Eye        Eye        eyes
Distance
 (mm)
 white
Distance
 (mm)
 black

								
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